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Irregular migration in the UK

11 Sep 2020

This briefing examines irregular migration – also called ‘illegal immigration’ – in the UK, including the difficulties in defining and measuring it, and evidence on its nature and scale.

  1. Key Points
    • There is no legal nor broadly accepted definition of an ‘irregular migrant’, though the term is most commonly used to refer to people who are in the UK without the legal right to be so.
    • Irregular migrants are not generally permitted to access benefits, social housing, or free hospital care.
    • Reliable and accurate estimation of the UK’s irregular migrant population has not to date been possible, so available estimates should be treated with caution.
    • Exit checks data do not provide an accurate measure of visa overstaying, and the government does not know with any degree of accuracy how many visa overstayers there are in the UK.
    • In the financial year 2017/18, around 1,800 people trying to enter the country without authorisation were apprehended in the UK, but the total number of attempted or successful unauthorised entries is not measurable.
    • Refused asylum seekers contribute to the UK’s irregular migrant population, but the precise number who have not left the country as required is not known.
    • In recent years, fewer people considered without the right to be in the UK were removed from the country or left voluntarily.
    • There are no statistics on how many people have been regularised through the 14 and 20 year pathways to regular status.
    • There is little evidence on whether amnesties increase future irregular immigration.
  1. A Note on Terminology ... Click to read more.

    Irregular migration is often referred to as illegal immigration. The use of the term illegal in this context is considered contentious, for two reasons. First, illegal immigration can suggest the breaching of criminal laws. In some countries, breaching immigration laws is an administrative rather than criminal matter (see FRA, 2014; Provera, 2015; Library of Congress, 2019). However, in the UK it is a criminal offence knowingly to enter or remain in the country without authorisation (Immigration Act 1971, Section 24), though prosecutions are rare.

    The second objection is to the related term illegal immigrant, which is argued to be degrading because it implies that people can be illegal. Most international organisations, including the United Nations and the International Organization for Migration, specifically avoid the terms illegal immigration and illegal immigrant for this reason, instead preferring irregular immigration and irregular migrant. The adjectives unauthorised and undocumented are also used to refer to the same phenomenon of illegal immigration, especially by researchers.

    The term undocumented migrant is commonly used as an alternative to illegal immigrant or irregular migrant. However, the term is often used ambiguously, sometimes to refer to migrants whose movement has not been documented (i.e., recorded), and other times to people without the required documents, such as a passport or valid visa. Importantly, an undocumented immigrant is not always an irregular migrant. A person might be in a country lawfully, but not possess the documents to prove so, such as certain members of the Windrush generation.

    Illegal immigration is also often used to refer to the unauthorised entry of people seeking asylum. However, because the claiming of asylum is not illegal, the term irregular is sometimes preferred to describe such movements.

    This briefing adopts the conventions of the UN and IOM in using the terms irregular migration and irregular migrant. It also refers to people who are in the UK without authorisation.

  1. Understanding the Policy

    In the UK, there is no single legal or accepted definition of an ‘irregular migrant’, also known as an ‘unauthorised’, ‘undocumented’, ‘non-registered’, or ‘illegal’ immigrant. ... Click to read more.

    However, a definition in common usage is a person who is in the UK without a valid residence permit. This definition is sometimes broadened to include migrants who have a valid residence permit, but who breach the conditions of their visa, such as by working without permission. However, it is more common to refer to such people as being ‘in breach’ of their visa conditions – although if that breach then leads to the revocation of their permission to say, they may then become an irregular migrant.

    Irregular migrants, defined narrowly as those in the UK without legal residence, are not permitted to work in the UK, claim benefits, or access some public services, such as university education, social housing, and most healthcare. Nor can they rent private accommodation, open bank accounts, or acquire driving licences. If an irregular migrant is found by the authorities, they risk being detained and removed.

    Regularisation: the 14- and 20-year routes to regular immigration status, and irregular migrant amnesties

    There are limited legal routes by which an irregular migrant can regularise their position and become a legal migrant. In the UK there used to be an immigration rule which allowed a person to qualify for indefinite leave to remain after 14 years of continuous residence in the country, including if some or all of that time was without legal residence. This rule was ended in July 2012 and replaced with the 20 year rule on long residence. Under this rule, if a person has lived in the UK for a continuous period of 20 years, including periods of unlawful residence, they can apply to remain in the UK and hence regularise their status.

    The UK has never introduced an amnesty for all irregular migrants (Lenoël, 2009, p. 106), though at least since 1974, the government has undertaken several programmes that granted legal status to various groups of people (see Levinson, 2005), including asylum seekers awaiting a decision on their claim (see our briefing on Settlement in the UK). Note that programmes granting status to pending asylum seekers are not technically regularisations, because the residence of the asylum seekers granted indefinite leave to remain was lawful on account of their outstanding asylum applications (Lenoël, 2009, p. 107).

    The ‘hostile environment’

    For many years, the UK government has aimed to reduce irregular migration. Its most recent set of policies with this aim is known collectively as the ‘hostile environment’ (now officially called the ‘compliant environment’), introduced mainly through the Immigration Acts 2014 and 2016. The stated purpose of this approach was to encourage irregular migrants to leave the UK of their own accord, and to deter people from becoming irregular migrants in the first place. The policy requires banks, landlords, employers and public service providers to refuse to provide services if the applicant could not demonstrate their legal immigration status.

  1. Understanding the Evidence

    The number and the characteristics of irregular migrants in the UK are unknown. Unlike aspects of legal migration, like visa grants or asylum claims, which are recorded and form a part of official published statistics, irregular migration is not recorded. ... Click to read more.

    Moreover, unauthorised entry, visa overstaying, illegal working, and violations of other visa restrictions, are all difficult for authorities to detect. As such, information on the UK’s irregular migrant population is indirect and piecemeal, making estimates of its size and composition highly uncertain. Further complicating estimation of the irregular population is that it is in constant flux, with many people cycling back and forth between regular and irregular immigration status.

    The residual method

    Most estimates of the UK’s irregular migrant population use the residual method. This is viewed as the best method and has been used in the estimations of several countries’ irregular migrant populations (e.g., Passel, 2007; Vogel and Kovacheva, 2008; Connor and Passel, 2019). The two most recent estimates of the UK’s irregular migrant population, by the Pew Research Center (Connor and Passel, 2019) and by Jolly et al. (2020) for the Greater London Authority, both used the residual method.

    The residual method begins with the idea that the total migrant population of a country – defined either as the foreign-born or foreign citizens – is made up of regular and irregular migrants. Therefore, subtracting the number of regular migrants from the total migrant population gives the number of irregular migrants. In the UK, the Census or Annual Population Survey (APS) are used to estimate the total migrant population, defined as those born abroad. This assumes that the population of irregular migrants will be captured by the household surveys that are used to estimate the total migrant population. However, this is unlikely, especially given that non-response is particularly high in inner city areas, where irregular migrants are more likely to live.

    A second limitation of the residual method is that it requires an accurate estimation of the regular migrant population. However, in the UK the population of legal migrants is not recorded and must be estimated using a range of assumptions that substantially affect the final estimate. For example, the residual method used by Woodbridge (2005) and adapted by the Greater London Authority (Jolly, et al., 2020) estimates the regular migrant population by adding migrants with permanent residence status to those with temporary residence status, minus estimates of the emigration and deaths of these groups, thus requiring assumptions to be made about these migrants’ emigration and mortality rates.

    Deportations, voluntary departures, exit checks, and unauthorised entries

    Unrelated to the residual method are data on what the Home Office calls returns: the enforced or voluntary departure of people whom it is considered have no legal right to be in the UK. As such, returns data provide partial and indirect evidence of the scale and composition of the irregular migrant population.

    Returns are of two broad types. Enforced returns refer primarily to people who left the UK, usually from detention, as a result of removal directions set by the Home Office. By contrast, voluntary returns are subject to a lower level of Home Office enforcement activity, or none at all. These include returns where the Home Office has facilitated or monitored the departure, including organising and paying for flights, and departures made without informing the authorities, which the Home Office has established after the fact (for more information on the different types of returns briefing on Deportation and Voluntary Departure from the UK). Importantly, data for the most recent year of voluntary returns are likely to be undercounts, because in some cases it can take time to identify people who have left the UK without informing the authorities. An unknown proportion of returns are due to migrants’ criminal behaviour. Such people could be argued to not be irregular because their stay was lawful until the deportation order was issued.

    Since April 2015, the Home Office has conducted checks of people leaving the UK and recorded whether they left before the expiry of their visa. These operational data look only at the people who were due to leave and do not include: those who had extended their visa; those whose expired visa was issued before the exit checks system was introduced; the majority of visitors to the UK who do not need a visa, such as UK and other EU nationals; and nationals from 52 countries whose citizens are not required to obtain a visa to visit for up to six months, who account for a large share of visitors to the UK. These data are currently classified as “Experimental Statistics”, which have a lower level of quality assurance than Official Statistics. The Home Office notes that these estimates “will not provide a full picture of the potentially resident illegal population” (Home Office, 2019e, p. 5). In part, this is because many departures are not recorded, for a variety of reasons (Home Office, 2016, p. 2). Therefore, these data indicate minimum levels of compliance amongst those whose visas have expired, and do not provide direct evidence of overstaying (see Home Office, 2019e; 2019f).

    In this briefing, data on detected unauthorised entries come from occasional inspections from the Independent Chief Inspector of Borders and Immigration (ICIBI) (e.g., Bolt, 2017; 2018), and data releases from the Home Office. Border Force do not routinely publish data on unsuccessful unauthorised entries, because “this could compromise immigration controls and impact on national security” (House of Commons, 2019).

There is no legal nor broadly accepted definition of an ‘irregular migrant’, though the term is commonly used to refer to people who are in the UK without the legal right to be so

There is no definition of an irregular migrant (or ‘illegal immigrant’) in UK law. However, it is commonly held that there are four main ways in which a person can become an irregular migrant (Figure 1):

  1. Enter the UK regularly and breach the conditions upon which entry or stay was granted, such as by visa overstaying, doing work that is not permitted, or due to a criminal conviction.
  2. Enter the UK irregularly or through deception, such as using forged documents or lying about the purpose of entry.
  3. Do not leave the country after an application for asylum has been rejected and all rights of appeal exhausted.
  4. Be born in the UK to parents who are irregular migrants, because the UK does not have birthright citizenship. (Some of these children can acquire citizenship directly. This category is included here because they are often included in estimates of the UK’s irregular population).

Many academic researchers believe that in the UK substantially greater numbers of people without regular immigration status arrived by air and entered regularly, rather than irregularly, and there is some evidence to support this (Düvell et al., 2018, p. 34).

In some definitions, the irregular migrant population includes those with legal residence but who work in breach of employment or other restrictions attached to their immigration status – a condition known as “semi-compliance” with immigration control (Ruhs and Anderson, 2010, p. 202). Such migrants are not typically included in estimates of the UK’s irregular migrant population.

Some people who claim asylum will have entered the UK irregularly. There is no asylum visa, which means that a person seeking asylum must either arrive irregularly or enter on a visa that has been issued for another purpose, such as tourism.

Figure 1

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Irregular migrants are not generally permitted to access benefits, social housing, or free hospital care

Irregular migrants are not permitted to access most social security benefits, including housing benefits, and some public services in the UK, such as social housing and hospital-based healthcare. Irregular migrants can access free primary healthcare care services, which includes GP and nurse consultations, and treatment provided by a GP. However, access to free secondary healthcare, which includes hospital treatment, is based on legal residence in the UK. As such, those without immigration status are generally charged for secondary healthcare services. There are some exceptions, including for emergency healthcare, treatment of communicable diseases, and for the diagnosis or treatment of coronavirus.

Other services, such as nursery schools and primary and secondary education (from the ages of 5 to 18), may in principle be accessed free, regardless of immigration status, although irregular migrants are often excluded indirectly. For example, many childcare services use eligibility for universal credit (which functions as a proxy for need) as a criterion for access, which in effect excludes irregular migrants.

One of the most extensive empirical studies of irregular migrants in the UK, based on 175 interviews (Düvell et al., 2018), found that while over half reported stress and anxiety resulting from their status, almost none were deterred by the ‘hostile environment’ policy. This study also found that access to irregular work depended on migrants’ ethnicity; and that some felt they were treated well by employers whilst others reported being paid below minimum wage, or being exploited, sometimes severely. A commonly cited reason for ensuring people use documented routes is to protect them from exploitation. A study of young undocumented migrants in the UK, based on 75 interviews, also found a lack of choice in the labour market, and dependence on work within ethnic-minority niches (Bloch et al., 2014).

Once a person’s status becomes irregular in the UK, it can be difficult for them to acquire regular status, in part because breaches of immigration law can result in bans from re-entering the country of between one and ten years (see Gbikpi, 2020).

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Reliable and accurate estimation of the UK’s irregular migrant population has not to date been possible, so available estimates should be treated with caution

Trying to estimate the number of irregular migrants in a country confronts the challenge of counting people who do not wish to be found. That is why attempts at estimation have been described as “counting the uncountable” (Kelly, 1977).

Nevertheless, there have been several estimates of the population of migrants living in the UK without a legal residence permit (Figure 2). These estimates do not include those with a legal residence permit, but who breach working or other restrictions.

Estimates vary in size, in part because they include different groups. Some include asylum seekers, or the UK-born children of irregular migrants – which one study estimated was 65,000 in 2012 (Sigona and Hughes, 2012) – while other estimates do not. But all estimates are highly uncertain, and have large margins of error.

The most recent estimates have been provided by the Pew Research Center (Connor and Passel, 2019) and Greater London Authority (Jolly et al., 2020). These were based on the residual method, with Pew producing an estimated range for 2017 of 800,000 to 1,200,000, and the GLA report providing a central estimate of 674,000 as at April 2017 (though this excludes the UK-born children of irregular migrants; if these are included the figure is 809,000).

These studies have important limitations. For instance, the Pew estimate of the lawfully resident migrant population was derived from a figure provided by the Home Office to Eurostat (Eurostat, 2020) that does not include (and is not intended to include) migrants with indefinite leave to remain (ILR). Excluding this group inflates the estimate by the number of migrants with ILR, a number that the government does not record, but which is likely to be in the hundreds of thousands. As noted in Understanding the Evidence, these estimates should be treated with caution. For a more detailed discussion of the limitations of the Pew and GLA methodologies, see our commentary Recent estimates of the irregular migrant population in the UK.

Figure 2

Two studies have provided estimates of the number of people in the EU who are without a valid residence permit (Vogel and Kovacheva, 2009; Connor and Passel, 2019). Both studies suggest that the UK had more irregular migrants than EU nations, but because the data available and methodologies used vary across countries, international comparisons should be treated with caution (Figure 3).

Figure 3

The next three sections of this briefing examine data on irregular migration ‘inflows’ by looking at three groups that add to the irregular migrant population: visa overstayers, unauthorised entrants, and refused asylum seekers.

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Exit checks data do not provide an accurate measure of visa overstaying, and the government does not know with any degree of accuracy how many visa overstayers there are in the UK

People who overstay their visas are a key source of the UK’s irregular migrant population.

In the last three financial years, there was an overall recorded compliance rate with visa duration of around 97%. In absolute terms, an average of around 55,000 non-EU visa nationals per year who were expected to leave the UK were not recorded as having left the country before their visa expired (this excludes non-visa visitors, such as US visitors; those who extended their visas; and those whose expired visa was issued before exit checks were introduced).

These exit check data can be broken down by visa type, to show that a large majority of expiries – 75% in this period – concerned people with visit visas, rather than study, work, or other (including family) visas (Figure 4). This reflects the much larger number of visitors who come to the UK compared to those on other visa types.

Importantly, because the departures of many people go unrecorded (see Home Office, 2019e; 2019f), these figures should be taken to represent the minimum level of compliance with visa duration for the categories covered, and are not an indicator of the rate of overstaying (see Evidence Gaps and Limitations). Nor is the UK’s population of visa overstayers known.

Of the 1.82m visa expiries in 2018/19, 0.4% were late departures (and 3.3% were not initially identified as departing). In 2017/18, 0.2% of expiries were late departures (Home Office, 2019e, p. 3).

Figure 4

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In the financial year 2017/18, around 1,800 people trying to enter the country without authorisation were apprehended in the UK, but the total number of attempted or successful unauthorised entries is not measurable

A 2018 report by the ICIBI stated that the number of unauthorised entrants apprehended by the Home Office “in country” was 2,366 in 2016/17, which declined by 23% to 1,832 in 2017/18 (Bolt, 2018, p. 26). In both of these financial years, Iraqi and Iranian nationals were the largest two nationalities apprehended.

Many attempts at unauthorised entry begin from French and Belgian ports where the UK implements immigration controls. At these ports, Border Force prevented around 53,000 attempts to enter the UK without authorisation in 2016/17, and around 28,000 in 2017/18 (partial fiscal year until 28 February), due to security improvements and migrant camp closures (Bolt, 2018, p. 30). These data refer to attempts to enter the UK rather than individuals apprehended. An individual may make repeated attempts at entry and hence be recorded more than once.

Since November 2018, there have been reports of an increase in migrants attempting to enter the UK irregularly by crossing the English Channel in small boats. Data on these small boat crossings are not routinely published. But on 3 September 2020, a senior official at the Home Office told the Home Affairs Committee that 5,000 people had made the crossing in small boats in 2020, 98% of whom claimed asylum (House of Commons, 2020, p. 10).

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Refused asylum seekers contribute to the UK’s irregular migrant population, but the precise number who have not left the country as required is not known

Asylum seekers who have been refused protection but do not leave the UK add to the irregular migrant population. Data on the outcomes of asylum applications, taking into account appeals, show that from 2010 to 2018, an average of around 9,500 asylum seekers were refused per year, while an average of around 4,000 were recorded as having departed the UK. This suggests that around 5,500 unsuccessful asylum seekers per year were not recorded as having departed the UK (whether via enforced, facilitated, or independent return) – equivalent to over half of all refused asylum applicants (Figure 5).

However, it is possible that many departures will not have been recorded for reasons other than overstaying, such as because the asylum applicant has been regularised, or has no entry record against which their departure can be matched. Therefore, these counts represent the upper bound of the number of refused asylum seekers who have become irregular migrants in the UK.

Figure 5

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In recent years, fewer people considered without the right to be in the UK were removed from the country or left voluntarily

From 2010 to 2019, an average of around 37,000 people without authorisation to remain in the UK left the country per year (Figure 6).

After 2015, returns fell year after year. This decline has been driven by falls in both enforced and voluntary returns, though the number of voluntary returns has fallen more steeply, though some of the fall, as explained in Understanding the Evidence, is due to initial undercounting.

From 2010 to 2019, voluntary returns have outnumbered enforced returns. The Home Office has a preference for voluntary returns (Bolt, 2015, p. 2), in part because its research shows that the average cost to the government of a voluntary return is £1,000, compared to £15,000 for an enforced return (Home Office, 2013, p. 4).

An increase in voluntary returns was a key objective of the government’s ‘hostile environment’ policy, introduced in 2014 and now officially known as the ‘compliant environment’ policy, which was designed to make remaining in the UK difficult for those without legal authorisation. However, it is unclear what impact the policy has had on voluntary returns: a 2020 report by the National Audit Office into the effectiveness of the hostile environment states that the Home Office is “currently unable to measure whether these activities have the desired effect of encouraging people to leave voluntarily” (National Audit Office, 2020, p. 28).

Figure 6

From March to July 2020, there has been no general government policy of suspending removals due to the coronavirus pandemic. However, new detentions of people liable to removal to 49 countries, including Jamaica, India, Pakistan, Afghanistan, Iraq, Sudan, and Albania have been stopped (Detention Action, 2020). As such, removals are likely to have reduced substantially. In May 2020, the BBC reported that around 50 people are believed to have been returned to their country of origin during the crisis. In 2019, an average of around 1,000 people were removed from the UK via enforced return each quarter.

For more information on the characteristics of returnees, see our briefing on Deportation and Voluntary Departure from the UK.

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There are no statistics on how many people have been regularised through the 14 and 20 year pathways to regular status

In response to a Freedom of Information request from the Migration Observatory on the number of people regularised through long residence – specifically, the 14 and 20 year routes – the Home Office stated:

“To produce a report or data relating in anyway [sic] to the numbers of applicants granted leave under the 14 year or 20 year route, we would have to manual trawl through every individual case record to obtain this data as it is not recorded in reportable field on our case management system.”

This suggests that the data exist, but until they have been compiled the government will not know how many people have had their status regularised due to long residence.

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There is little evidence on whether amnesties increase future irregular immigration

A few academic studies address the question of whether amnesties increase irregular immigration in the future by acting as a ‘pull factor’. Most research focuses on the United States and examines the effects of the 1986 Immigration Reform and Control Act, which granted amnesty to nearly 2.7 million undocumented migrants (e.g., Bean et al., 1990; Donato et al., 1992). The most recent study found that the Act did not change patterns in long-term undocumented immigration, which is driven by several factors, most notably economic conditions (Orrenius & Zavodny, 2003). One research project examined regularisations across the EU, and concluded similarly that, “[t]he existing literature, and our own research, provides no evidence of the ‘pull factor’ for regularisation programmes: the situation is far more complex, and involves many more variables which are typically not under political control” (Baldwin-Edwards & Kraler, 2009, p. 131).

Evidence Gaps and Limitations

There is a lack of good data for the accurate estimation of the irregular migrant population of the UK. This is in part why ONS and the Home Office published a joint statement in 2019 suggesting they did not plan to produce a new estimate using the residual method.

Returns data are limited to whether the returnee previously made an asylum claim or is a foreign national offender. There is no information regarding the immigration history of returnees, and so no data on how many of those returned entered without authorisation, had their application for asylum refused, overstayed their visa, or who otherwise breached the conditions of their leave to remain in the UK.

However, returnees are just one part of the picture, which could be better illuminated if there were more data on the journeys of people into, and out of, irregular status.

With thanks to Jaqueline Broadhead, Sarah Spencer, John Salt and CJ McKinney for invaluable feedback on drafts of this briefing.

This briefing was made possible with the support of the Joseph Rowntree Charitable Trust.


Related material

Migration Observatory briefing – Deportation and Voluntary Departure from the UK

Migration Observatory briefing – Migrant Settlement in the UK


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