This briefing examines unauthorised migration—also known as ‘illegal immigration’ or ‘irregular migration’—in the UK, including the difficulties in defining and measuring it and evidence on its nature and scale.
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Key Points
- There are four main ways for a person to become an unauthorised migrant in the UK: enter regularly on a visa and overstay, enter without authorisation, remain after asylum application avenues are exhausted, or be born in the UK to irregular migrant parents.
More… - People who migrate without authorisation are typically aware of the risks and benefits of doing so but face limited information and possible bias.
More… - The number of people detected arriving in the UK without permission has increased in recent years, mostly after crossing the English Channel in small boats.
More… - Refused asylum seekers contribute to the UK’s unauthorised migrant population, though the precise number who remain in the country is uncertain.
More… - The government does not know with any accuracy how many visa overstayers there are in the UK.
More… - Returns of people without a legal right to be in the UK have partly recovered in recent years after a period of decline.
More… - Unauthorised migrants are not permitted to access jobs, housing, benefits, and most free public services in the UK.
More… - It is not known how many unauthorised migrants have been regularised in the UK.
More…
- There are four main ways for a person to become an unauthorised migrant in the UK: enter regularly on a visa and overstay, enter without authorisation, remain after asylum application avenues are exhausted, or be born in the UK to irregular migrant parents.
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Understanding the Policy
Different terms are used to describe the movement of people across borders without permission, including illegal, irregular, or unauthorised migration. ... Click to read more.They mean broadly the same thing. In the UK, it is a criminal offence to knowingly enter or remain in the country without authorisation (Immigration Act 1971, Section 24), although criminal prosecutions are rare. Official statistics published by the Home Office and Office for National Statistics typically use the term ‘irregular migration’. However, this briefing uses the term ‘unauthorised’ because research with members of the public has suggested that public audiences find it to be clearer and more impartial than either ‘irregular’ or ‘illegal’.
The term undocumented migrant is commonly used as an alternative to illegal immigrant, unauthorised migrant or irregular migrant. However, ‘undocumented’ can be ambiguous, referring either to migrants whose movement has not been documented (i.e., recorded) or other times to people who have the right to live in the UK but lack the documents to demonstrate it (e.g. some members of the Windrush generation).
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 – a condition known as “semi-compliance” with immigration control. Such migrants are not typically included in estimates of the UK’s unauthorised migrant population unless those violations resulted in the withdrawal of legal residence.
Unauthorised migrants are not permitted to work in the UK, claim benefits, or access some public services, such as university education, social housing, and secondary healthcare (post-GP hospital or specialist care). Nor can they rent private accommodation, open bank accounts, or acquire driving licences. If an unauthorised migrant is found by the authorities, they risk being detained and removed.
Regularisation and amnesties
People who have lived in the UK for a continuous period of 20 years, including periods of unlawful residence, can apply to remain in the UK and hence regularise their status (before July 2012, this period was 14 years). Some unauthorised migrants can also apply for residence rights on the basis of their family or private life in the UK. For more information, see the Migration Observatory’s briefing, Migrants on ten-year routes to settlement in the UK. Some children without legal residence rights can apply directly for UK citizenship, including if they are born in the UK and lived here for the first ten years of their life.
The UK has never introduced an amnesty for all unauthorised migrants, though at least since 1974, the government has undertaken several programmes that granted legal status to various groups of people, including asylum seekers awaiting a decision on their claim (see our briefing on Settlement in the UK).
The ‘hostile environment’
For many years, the UK government has aimed to reduce unauthorised migration. In the early 2010s, a set of policies was introduced with this aim, known collectively as the ‘hostile environment’ (now officially called the ‘compliant environment’). They were introduced mainly through the Immigration Acts 2014 and 2016. The policies required banks, landlords, employers and public service providers to refuse to provide services if the applicant cannot demonstrate their legal immigration status. The stated purpose of this approach was to encourage unauthorised migrants to leave the UK of their own accord and to deter people from becoming unauthorised migrants in the first place.
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Understanding the Evidence
The number and the characteristics of unauthorised migrants in the UK are largely unknown. Unauthorised entry, visa overstaying, illegal working, and violations of other visa restrictions, are all difficult for authorities to detect. ... Click to read more.Therefore, information on the UK’s unauthorised migrant population is indirect and piecemeal, making estimates of its size and composition highly uncertain. The Home Office has stated that “It is not possible to know the exact number of people currently resident in the UK without permission, nor the total number of people who enter the UK irregularly, and so we do not seek to make any official estimates of the illegal population.” Further complicating the estimation of the unauthorised migrant population is that many people cycle back and forth between regular and irregular immigration status.
Home Office statistics on irregular migration to the UK
Despite these challenges, in 2022, the Home Office began publishing statistics on irregular migration as part of its regular quarterly statistical releases. These statistics focus on the number of detected unauthorised arrivals to the UK, including via small boat. The data count the number of detections rather than the number of unique individuals so that if a person is detected entering the UK without authorisation more than once in a given period, they are counted more than once in the statistics. The Home Office cautions that its detection statistics should not be used to infer actual patterns of overall unauthorised migration because they are likely to be affected by recording practices and operational activity at the border and overseas, such as a changing number of patrols or vehicle checks.
The residual method
Most estimates of the UK’s unauthorised migrant population use the residual method. This is viewed as the best method and has been used to estimate the unauthorised migrant populations of several countries. The two most recent estimates of the UK’s unauthorised migrant population, by the Pew Research Center and by researchers working 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 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 unauthorised 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 unauthorised 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 unauthorised 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 the Greater London Authority study 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 to 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 unauthorised 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 details, see our 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 unauthorised because their stay was lawful until the deportation order was issued.
After 2015, the Home Office conducted checks of people leaving the UK and recorded whether they left before the expiry of their visa. These operational data looked only at the people who were due to leave and did not include those who had extended their visas, those whose expired visa was issued before the exit checks system was introduced, or the majority of visitors to the UK who do not need a visa, who account for a large share of visitors to the UK.
These data were classified as “Experimental Statistics”, which have a lower level of quality assurance than Official Statistics. The Home Office noted that these estimates “will not provide a full picture of the potentially resident illegal population”. In part, this is because many departures are not recorded for a variety of reasons. Therefore, these data indicated minimum levels of compliance amongst those whose visas have expired and did not provide direct evidence of overstaying.
However, the Home Office discontinued these statistics after 2020 and said it would review the combined impact of the pandemic and Brexit on the accuracy of its figures in the future. As of January 2025, no updates had been provided on this process, and no recent statistics on exit checks were publicly available.
There are four main ways for a person to become an unauthorised migrant in the UK
There are four main ways in which a person can enter the population of unauthorised migrants, i.e. people resident in the UK without permission:
- Entering the UK on a visa and overstay (including in cases where residence permission is cancelled due to a criminal conviction).
- Entering the UK without authorisation or through deception, such as using forged documents.
- Not leaving the country after an asylum application has been rejected and all rights of appeal exhausted.
- Being born in the UK to parents who are unauthorised migrants because the UK does not have birthright citizenship.
Figure 1
Research indicates that many unauthorised migrants first arrived in the UK legally, usually by air, rather than without authorisation. Some people who claim asylum will initially have entered the UK without authorisation or lost legal status after overstaying a visa. People with pending asylum claims have some legal protections (most notably, against being removed to their country of origin) and thus tend to be described as “asylum seekers” and not “unauthorised migrants”.
People who migrate without authorisation are aware of the risks and benefits of doing so but face limited information and possible bias
Research has tackled several key questions about unauthorised migrants’ decision-making.
What makes people migrate without authorisation? The reason is often seen as a combination of push factors which make them leave their home country – such as political instability, violence, or economic turmoil – and pull factors which attract them to destination societies, like respect for human rights or better economic opportunities. Networks of existing migrants in other countries also play a big role in shaping migration aspirations and spreading information about routes and destinations.
Research has found that for some groups of migrants, such as asylum seekers, push factors often play a bigger role in individuals’ decision to migrate without authorisation, especially compared to the pull of specific destination countries. In some cases involving asylum seekers, the choice of destination is left to smuggling agents, and migrants are particularly reliant on their decisions.
Are people aware of the risks and benefits of unauthorised migration? To some extent, yes. Prospective migrants are generally aware of both the risks involved in an unauthorised journey – such as death, violence, or abuse – and the potential economic benefits of reaching their destination. However, they face great uncertainty and limited information – many underestimate risks and overestimate their chances of obtaining legal status. In addition, few have detailed knowledge of recent developments in destination countries. Studies looking at asylum seekers also found they were relatively uninformed about welfare entitlements or policy changes at destination.
Available estimates of the UK’s unauthorised migrant population should be treated with caution
It is difficult to estimate the number of unauthorised migrants in a country. Attempts at estimation have been described as ‘counting the uncountable’ since they confront the challenge of counting individuals who do not wish to be found. Nevertheless, there are several estimates of the population of migrants living in the UK without a legal residence permit. These do not include those with a legal residence permit but who breach working or other restrictions.
The most recent estimates are for 2017 and were separately produced by the Pew Research Center and the Greater London Authority, both using the residual method (see Understanding the Evidence). The first shows an estimated range of 800,000 to 1.2 million unauthorised migrants in the UK, while the GLA’s central estimate stood at 674,000 – going up to 809,000 when including UK-born children of unauthorised migrants.
Figure 2
In part, estimates vary because they include different groups. Some include asylum seekers or children born in the UK to unauthorised migrant parents, while others do not. However, all figures are highly uncertain and have large margins of error. For example, the Pew estimate of the lawfully resident migrant population was derived from figures provided by the Home Office to Eurostat which exclude migrants with indefinite leave to remain (ILR). This 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. All estimates provided above should thus be treated with caution. For more details, see the Migration Observatory’s commentary Recent estimates of the irregular migrant population in the UK.
According to one study that analysed and compiled existing estimates of the unauthorised population in multiple countries, the UK had the second highest number of unauthorised migrants in Europe, after Germany. Two previous analyses conducted across the EU in 2008 and 2017 also showed the UK had more unauthorised migrants than most other European countries (Figure 3). However, the available data and methodologies used vary substantially between countries, meaning that international comparisons are highly uncertain, and any clear conclusions should be treated with caution.
Figure 3
The next three sections of this briefing examine data on unauthorised migration ‘inflows’ by looking at three groups that add to the unauthorised migrant population: unauthorised entrants, refused asylum seekers, and visa overstayers.
More people were detected arriving in the UK without authorisation in recent years, mostly after crossing the English Channel in small boats
The number of people detected arriving in the UK without authorisation has increased sharply in recent years, driven by the rise of the small boats route across the English Channel. Between 2020 and the end of September 2024, around 175,000 unauthorised arrivals were recorded by the authorities. Of these, 78% arrived in the UK after crossing the Channel in a small boat.
While reports of people attempting to enter the UK by crossing the Channel go back over fifty years, significant numbers did not begin to be detected until late 2018. From 2020, numbers increased rapidly. Several reports suggest that the initial emergence of the small boats route was a consequence of increased border enforcement that made it significantly harder for people to arrive in the UK through other irregular means, including as stowaways on trains and lorries. British and French authorities invested heavily in border security after 2014 to prevent such journeys. Growing experience and the professionalisation of smuggling operations are likely to have contributed to a significant expansion of the route.
Figure 4
Most people who arrive in the UK by small boat subsequently apply for asylum – around 94% of those who arrived between 2018 and the end of September 2024. Of those who received an initial decision, around 70% were granted protection, similar to the grant rate for all asylum applications.
For more details about small boat migration to the UK, see the Migration Observatory’s briefing, People crossing the English Channel in small boats.
Refused asylum seekers contribute to the UK’s unauthorised migrant population, though the precise number who remain in the country is uncertain
Asylum seekers who have been refused protection but remain in the UK add to the unauthorised migrant population. Official data show that 166,000 people applied for asylum between 2010 and 2023 but were refused protection, taking into account appeals. Of these, around 82,000 were recorded as having left the UK via enforced or voluntary return by 30 June 2024. This suggests that roughly half of refused asylum seekers during the period, around 85,000 individuals, had not been recorded as having departed the country.
However, some 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 unauthorised migrants in the UK.
Figure 5
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 unauthorised migrant population.
However, there is a lack of data on how many visa overstayers remain in the UK, particularly in recent years. The Home Office discontinued its statistics related to exit checks after 2020 to review the impact of Brexit and the pandemic on the accuracy of these figures. At the time of writing in January 2025, no further updates had been provided on their replacement.
In the four years to March 2020 – the last for which data were available – an average of around 63,000 non-EU visa nationals a year were not recorded as having left the country before their visa expired (Figure 6). This marks an overall recorded compliance rate of around 96%.
Yet many departures from the UK either go unrecorded or cannot be matched against people’s initial arrival in the system. These figures, hence, represent the minimum level of compliance with visa duration and are not an indicator of the rate of overstaying (see Evidence Gaps and Limitations). The UK’s population of visa overstayers is also not known.
Figure 6
Returns of people without a legal right to be in the UK have partly recovered in recent years after a period of decline
In the year ending 30 September 2024, around 31,500 people were returned from the UK, either forcibly or voluntarily. This was 19% higher than in 2013 and the highest level since 2017. The number of returns fell sharply during the 2010s and has only partly recovered (Figure 7).
The Home Office prefers voluntary returns, which are much cheaper and are considered more humane. An estimate from 2013 put the average cost of an enforced return at £15,000. In contrast, an average voluntary return cost of around £7,000 in 2015.
It is unclear exactly why returns declined so much in the 2010s. Potential explanations include higher costs and fewer resources dedicated to enforcement, legal challenges to removal, administrative issues in the Home Office, and difficulties in securing cooperation from countries of origin. The new Labour government has pledged to significantly increase returns in the coming years (we are tracking whether this and other migration pledgees are being met in another commentary).
For more details, see the Migration Observatory’s briefing, Deportation and Voluntary Departure from the UK.
Figure 7
Unauthorised migrants are not permitted to access jobs, housing, benefits, and most free public services in the UK
Unauthorised migrants face strict restrictions on their rights in the UK, particularly after the introduction of ‘hostile environment’ policies in the early 2010s. This means that people without legal status cannot get a job, rent a house, open a bank account, or have a driving licence in the UK. In addition, unauthorised migrants are not permitted to access social security benefits like Universal Credit or key public services like social housing. Free access to specialist or hospital treatment is also limited to those with legal residence in the UK (see Understanding the Policy).
There is some evidence from the United States that employment restrictions made employers less likely to hire unauthorised workers and made those who had left the country less willing to return. However, interviews with migrants who remain in the UK despite the restrictions also suggest that migrants are often able to adapt their strategies to avoid detection. This may help to explain why a majority of migrants sanctioned under hostile environment rules from 2015 to 2018 were still in the UK a few years later, according to a Home Office evaluation. A Home Office review of the evidence from the UK and other countries in 2023 concluded that it was not clear to what extent compliant environment measures had the desired effect of deterring unauthorised migrants or encouraging them to leave the UK. This does not mean there is no impact, as policy impacts in this field can be difficult to measure.
Research has also suggested that hostile environment restrictions can have unintended consequences, such as leaving unauthorised migrants more vulnerable to exploitation from employers or encouraging discrimination against legal migrants in the rental market.
It is unclear how many unauthorised migrants have been regularised in the UK
The Home Office does not publish statistics on how many unauthorised migrants it has given legal status to.
One path to regularisation is having continuously lived in the UK for a very long period, despite not having legal permission to do so – currently 20 years (see Understanding the Policy). It is unclear how many people become regularised through these routes. In response to a Freedom of Information request, the Home Office stated that it was not able to extract statistics from its management systems.
People can also apply for the right to remain in the UK based on a human rights claim under the ECHR, which protects the right to private and family life. UK law directs judges to place ‘little weight’ on a private life or relationship formed while someone was in the UK unlawfully, though some unauthorised migrants may be granted status under this route. The Home Office does not publish data on the number of human rights-related regularisations. Between 2015 and 2023, an average of around 19,000 people with no previous recorded status entered the “family life” route, though many of them will have switched from another legal status (which was not recorded due to administrative issues).
There are other ways to gain legal status, such as being granted asylum or humanitarian protection. For more information, see our briefing on Asylum and Refugee Resettlement in the UK.
Research is unclear on whether giving unauthorised migrants legal status increases irregular immigration in the future. Most studies are based on the US, where the evidence shows no impact of amnesties on long-term patterns of undocumented migration. Another project examining the effect of regularisation programmes in the EU similarly did not find an impact on future irregular arrivals. As a result, unauthorised populations continued to develop after the amnesties took place.
Evidence Gaps and Limitations
There are many data gaps in unauthorised migration in the UK.
There are limited data to estimate the unauthorised 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.
In other areas, data in theory exist but are not published. For example, there have been no recent statistics on visa overstayers or grants of legal status to previously unauthorised migrants.
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 otherwise breached the conditions of their leave to remain in the UK.
Acknowledgements
With thanks to Jaqueline Broadhead, John Salt and CJ McKinney for invaluable feedback on previous versions of this briefing.
