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Pew Research Centre Estimates on the Irregular Migrant Population the UK and the rest of Europe

13 Nov 2019

The Pew Research Centre has produced new estimates of the number of irregular (‘illegal’ or ‘unauthorised’) migrants in the EU, including the UK. Here we explain briefly what they find and how they reach their conclusions.

What are the key findings for the UK?

The report estimates that in 2017 there were between 800,000 and 1.2m people living in the UK without a valid residence permit. The authors also estimate that, in 2017:

  • Around one third of irregular migrants had been living in the UK for 10 years or more;
  • They included similar shares of men and women, and around 14% were children;
  • There was no evidence of any increase in the number of irregular migrants living in the UK since 2014;
  • Half came from the ‘Asia Pacific’ region, but there no breakdown by individual countries within that region;
  • The UK had one of the largest irregular migrant populations in Europe, alongside Germany.

How are the figures calculated and are they accurate?

The study uses the ‘residual method’. It compares the estimated the number of non-EU citizens living in the UK to an estimate of the number holding a valid residence permit in the same year.

The results come with a high degree of uncertainty, because both of these figures are just estimates—as the Pew report recognises.

In 2017, ONS estimated that 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 agrees to respond.

Separately, the Home Office is required to report to Eurostat an estimate of the number of the non-EU citizens holding a valid residence permit each year – ranging from temporary work permit holders to long-term residents with Indefinite Leave to Remain (ILR). In 2017, this estimate was roughly 1.5m. However, the UK government does not actually know the precise number of legally resident non-EU citizens, and the estimate requires various assumptions, for example about how many people with ILR have left the country or died.

Pew’s ‘lower-bound’ estimate of 800,000 compares the estimated non-EU citizen population with the number of valid residence permits of at least 3 months duration. The ‘upper bound’ estimate of 1.2m instead looks only at 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 here.

The comparison between the UK and other EU countries is particularly uncertain because the estimates of the number of legal residents are produced in very different ways and are not thought to be comparable.

In summary, without more accurate data on both the number non-EU citizens in the UK and the number holding valid residence authorisation, there will continue to be a high degree of uncertainty around the number of people living here without authorisation.

Earlier this year, ONS and the Home Office produced a joint statement suggesting they did not plan to produce a new estimate using this method, because of limitations in the data and methodologies.

What are the remaining evidence gaps?

Even if we cannot be certain about the number, it is reasonable to assume based on this and previous estimates that the UK has a substantial irregular migrant population. There are still many things that are not known about the unauthorised population, notably:

  • It is not known how many entered illegally vs. came legally but later overstayed or were not able to renew their residence authorisation.
  • The figures do not tell us what the impacts of policy have been on the decisions irregular migrants make, and/or whether the figure would have been higher or lower if different policies had been in place.

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