What we do (and don’t) know about ‘illegal migrant’ numbers

A front-page article published in the Daily Telegraph today claiming that “London is home to as many as 585,000 illegal migrants, equivalent to one in 12 of the city’s population” has been widely covered in the media and led to debate and commentary from politicians and others online. We’ve had readers get in touch to ask about the figures.

We’ve taken a look at how the figure was calculated and what it’s based on—and why experts say estimates of “illegal migrant” populations should be treated with caution.

Where does the ‘up to one in 12’ figure come from?

The article cites a report conducted by the research company Edge Analytics for Thames Water in February 2023, which the Telegraph says it obtained “under freedom of information-style laws for the environment”. The Edge Analytics report, which Full Fact has seen but which doesn’t appear to have been published online, produced estimates for the number of “hidden and transient” users of Thames Water’s services. Edge Analytics defines this group as including irregular migrants (people in the UK without legal residence) as well as people living at secondary addresses, visitors and tourists.

The report bases its estimate for the number of irregular migrants in London on a study published by the Pew Research Center which estimated that the total number of irregular migrants in the UK in 2017 was between 800,000 and 1.2 million.

To calculate the number of irregular migrants in London, the report then considered government data on the number of National Insurance number registrations allocated to non-EU foreign nationals in local authorities between 2011 and 2019, as well as other studies which it said suggested that around 60% of the UK’s irregular migrants live in London. It’s not entirely clear from the report how this calculation was done however—we’ve asked Edge Analytics about this and will update this article if we hear back.

The Telegraph article’s claim that “up to one in 12 of the capital’s population is an illegal migrant” is based on the report’s upper-range estimate of the number of irregular migrants in London (585,533) as a proportion of the report’s estimate of the “usual resident population” of the city in 2021 (7,044,667). It’s not immediately clear where this population estimate comes from—other estimates of the London population in 2021 are substantially higher.

However, this “usual resident population” figure does not include the estimated 585,533 irregular migrants, and it’s unclear why the Telegraph appears not to have added the figure for irregular migrants to the city’s population when calculating its ‘up to one in 12’ figure—if it had, then the proportion of London’s population who are irregular migrants would have been slightly lower (about one in 13). We’ve asked the Telegraph why it calculated its figure in this way and will update this article if we receive a response. 

Who are ‘irregular migrants’ and what do we know about them?

The University of Oxford’s Migration Observatory published a detailed briefing on “unauthorised” or irregular migrants earlier this week, and says they broadly belong to four categories:

• People who have overstayed their visa
• People who enter the UK without authorisation or through deception
• Rejected asylum applicants who do not leave the UK
• Children of unauthorised migrants who are born in the UK

Not all of these people will have always been in the UK irregularly (for example, some may have entered on legitimate visas, while people with pending asylum applications are also not necessarily considered ”irregular migrants”). 

Estimating the actual number of people in these groups is inherently difficult, as many irregular migrants are likely to want to avoid detection by authorities.

The Migration Observatory warns all estimates of the UK’s irregular migrant population should be treated with caution, noting that “all figures are highly uncertain and have large margins of error”.We’ve asked the Migration Observatory about the figure published in the Telegraph today and will update this article if we hear back. 

What do recent estimates show?

According to the Migration Observatory, the two most recent studies looking at the UK’s irregular migrant population were conducted by the Pew Research Center (the report on which Edge Analytics’ figures were based in part) and the Greater London Authority. Both proposed estimates based on 2017 figures, so may now be out of date.

As noted above, the Pew Research Center estimated that in 2017 the number of unauthorised migrants (not including asylum seekers with pending applications) across the UK was between 800,000 and 1.2 million. It did not estimate how many of these were living in London.

The Greater London Authority estimated that in 2017 there were 674,000 “undocumented individuals” in the UK, 397,000 of whom lived in London.

But again, it’s important to be clear that these figures are estimates and should be treated with caution. 

Neither the Home Office nor the Office for National Statistics (ONS) currently publish estimates of the UK’s irregular migrant population. In a 2019 report, the ONS said: “By its very nature, it is extremely difficult to know the exact size of the illegally resident population and due to the challenges in making reliable estimates the government has not produced any official estimates since the mid-2000s.”