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Recent estimates of the UK’s irregular migrant population

11 Sep 2020

By Peter William Walsh and Madeleine Sumption

In late 2019 and early 2020, new estimates of the UK’s irregular migrant population were published. The first was in the report, Europe’s Unauthorized Immigrant Population Peaks in 2016, Then Levels Off, published by the Pew Research Center. Its authors, Phillip Connor and Jeffrey S. Passel, produced new estimates of the number of irregular (or illegal, unauthorised, undocumented) migrants living in the European Union, including the United Kingdom.

The second estimate was in the report, London’s Children and Young People Who Are Not British Citizens: A Profile, produced for the Greater London Authority (GLA) by Andy Jolly, Sián Thomas and James Stanyer, researchers at the University of Wolverhampton. This study provided an estimate for the UK only.

This commentary examines the findings of the Pew and GLA studies. It also looks at how these findings were produced, and highlights their high degree of uncertainty.

For more information on irregular migration in the UK, see the Migration Observatory briefing, Irregular Migration in the UK.

What were the key findings?

The Pew report estimates that at the end of 2017 there were between 800,000 and 1.2m people living in the UK without a valid residence permit, equivalent to between 1.2–1.8% of the UK’s resident population of 65m. The authors also estimate that at the end of 2017:

  • the UK had one of the largest irregular migrant populations in Europe, alongside Germany, accounting for roughly a quarter of the total irregular migrant population of the 32 European Union and European Free Trade Association (EFTA) countries;
  • half of the UK’s irregular migrants came from the ‘Asia Pacific’ region, and a fifth from sub-Saharan African countries, though no breakdown by country is provided;
  • the irregular migrant population was roughly equally male and female;
  • most (58%) were under 35, and 14% were under 18;
  • around one-third (36%) of irregular migrants had been living in the UK for ten years or more.

The authors also found that the UK’s irregular population was relatively constant from 2014 to 2017; and suggest that most of this population is likely to be made up of visa overstayers and asylum seekers who have not left the UK after having their claims rejected.

The GLA report provides central estimates with margins of error, rather than estimated ranges like the Pew report. It estimates that at the beginning of April 2017 there were 674,000 irregular migrants living in the UK, equal to 1% of the population, though this does not include the UK-born children of irregular migrants. The GLA researchers also provide the following central estimates:

  • if the UK-born children of irregular migrants are included in the estimate of the UK’s irregular population, the figure is 809,000;
  • 397,000 of all the UK’s irregular migrant adults and children live in London, including the UK-born children of irregular migrants;
  • there were 215,000 undocumented children in the UK (including the UK-born);
  • over half (59%) of all the UK’s irregular migrant children live in London;
  • around half of all irregular migrant children were born in the UK to irregular migrants.

How were the estimates calculated and are they accurate?

Both studies use the same broad methodology, the residual method, which is considered the best methodology for estimating irregular migrant populations. The residual method is based on the idea that a country’s total migrant population – defined as either foreign citizens or the foreign-born – is made up of both regular migrants and irregular migrants. Subtracting the number of regular migrants from the total migrant population therefore gives the number of irregular migrants.

Although both studies use this methodology, there are important differences between the two. Notably, Pew defines migrants based on their citizenship (i.e., being non-British, non-EU, or non-EFTA), whilst the GLA researchers use country of birth. EU citizens are excluded from both studies because they did not require any form of permit to be lawfully resident in the UK.

The Pew estimates

The Pew researchers compare the estimated total number of non-EU citizens living in the UK with an estimate of those holding a valid residence permit in the same year. The gap between these numbers is the estimated irregular migrant population. The results come with a high degree of uncertainty, because precise statistics on either of these groups are not available.

The Office for National Statistics (ONS) estimated that in 2017 there were around 2.4m non-EU citizens living in the UK. (This is lower than the 5.7m non-EU-born migrants living in the UK that year, because most people born in non-EU countries now hold UK citizenship.) The precise figure is uncertain for various reasons, including because it is drawn from a statistical survey to which not everyone agreed to respond.

Importantly, there are no accurate data on the number of lawfully resident non-EU citizens. The Home Office is required each year to report to Eurostat an estimate of the number of non-EU citizens holding a valid residence permit. In 2017, this estimate was roughly 1.5m. This estimate is based on the number of temporary residence permits issued that remain valid and includes, for example, visas for entry and extensions where these have been granted.

However, the major problem with using this figure as an estimate of the legally resident population is that it does not include migrants who have been granted permanent residence (i.e., indefinite leave to remain, also known as settlement).

How many migrants with ILR are there in the UK? The government does not know, but Home Office data allow us to estimate the number of people who were granted visas in each year from 2004 to 2017 who had ILR but not citizenship by the end of 2017. In this 14-year period, around 283,000 non-EU citizens were granted ILR but did not go on to become citizens (Table 1). Some of these people will since have left the UK, and it is not known how many are still resident. (The 581,000 people granted ILR followed by citizenship by the end of 2017 are not included in Pew’s analysis because the report only considers non-EU citizens.)

The number who received ILR before 2004 is not known, but could be in the hundreds of thousands. ONS population data suggest that in 2017, around 250,000 non-EU citizens had been living in the UK since before 1993, that is, for more than 25 years. The large majority of these long-term residents will have ILR that was granted before 2004.

Table 1


In addition, any estimate of the number of legally resident non-EU citizens requires various assumptions, for example about how many people with ILR have left the country or died.

Pew’s low estimate of 800,000 compares the estimated non-EU citizen population with the number of people with valid residence permits of at least 3 months’ duration. The high estimate of 1.2m instead includes only those with permits lasting at least a year, and also adjusts the figure upwards to account for the possibility that ONS has underestimated the number of non-EU citizens living in the UK.

The Pew report uses different methodologies for different countries. The residual method was used for 11 of the 32 countries studied. International comparisons should therefore be made with caution. However, even where the residual method is used across countries, comparing the estimates of the UK’s unauthorised population with those for other countries remains problematic, because the estimates of the number of legal residents are produced in very different ways.

These issues, especially Pew’s exclusion of legal migrants with ILR from its estimate of the legally resident non-EU population, raise significant questions about the accuracy of the estimates. However, it is critical to note that simply subtracting any of these numbers from Pew’s estimates does not improve their robustness. There are other factors that also contribute to uncertainty, such as the possibility that official population figures undercount non-EU citizens. Without better data on both the number of non-EU citizens in the UK and the number with valid residence, there will continue to be a high degree of uncertainty around the number of people living here without authorisation.

In part, that is why in 2019 ONS and the Home Office produced a joint statement suggesting they did not plan to produce a new estimate of the UK’s irregular migrant population using the residual method, because of methodological and data limitations.

How were the GLA estimates calculated?

The GLA methodology is more complex, as it attempts to estimate the number of legally resident migrants by looking at the size of several different sub-groups within this population (instead of using the problematic Eurostat number). Unlike the Pew estimate, the GLA’s does account for people with settlement, using Home Office data. This, broadly speaking, replicates the approach used by Woodbridge (2005), which was used to estimate the size of the irregular population in 2001.

The GLA researchers therefore calculate their estimate by summing the number of permanent and temporary legal migrants, adjusting for emigration and deaths, and subtracting this figure from an estimate of the total foreign-born population derived from the Annual Population Survey and the UK’s census, which is conducted every ten years. Like the Pew estimate, several assumptions are required that could substantially affect the final estimate.

Their headline figure of 674,000 irregular migrants or 809,000 including the UK-born children of migrants, is broadly similar to the Pew central estimate. The main reason for this is that while the GLA researchers do account for people with settlement, they also assume that the foreign-born population of the UK is larger, adding a (necessarily arbitrary) 10% to the ONS estimate in order to compensate for expected undercounting.

Concluding remarks

We cannot be certain about the number of irregular migrants living in the UK. Yet it is reasonable to assume, based on these and previous estimates, that the UK has a substantial irregular migrant population.

There are still many things that are not known about this population. From a policy perspective, some of these things will be more important than its size. For example, we do not know how people become irregular migrants – for example, how many entered illegally versus how many entered legally but later overstayed. Nor do these estimates tell us what the impacts of policy have been on the behaviour of irregular migrants, or whether their numbers would be higher or lower if different policies had been in place.

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

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