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

A front-page article published in the Daily Telegraph 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.

These figures are estimates which rely on a number of assumptions. Experts say any such estimates of “illegal migrant” populations should be treated with caution.

Here’s what we know about the figures quoted in the Telegraph, and what other data is out there.

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. Edge Analytics told us it wasn’t able to provide further details under its contract with Thames Water, but said it had “concerns” about how the information and data in its report was presented by the Telegraph. Edge Analytics said it “continues to apply an academic rigor to all of its demographic analysis projects”.

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). 

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). 

It’s also not entirely clear from the Telegraph article where this population estimate comes from—according to the 2021 Census London’s usual resident population is 8,799,800. A correction published by The Times on 28 January stated that the seven million figure is based on Thames Water’s London “water resource zone”, which does not cover all of Greater London and includes some areas outside London, though we’ve not been able to verify this directly.

If this is what the seven million figure is based on, this is not made clear in the Telegraph article, which refers to “the whole of London”.

Finally, it’s worth noting that the Telegraph’s figure is an upper-range estimate—this is made clear in its online headline, which claims “up to one in 12” people in London is an “illegal migrant”, but not in its print headline, which states “one in 12 in London is illegal migrant”. 

We’ve contacted the Telegraph for comment 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”.The Migration Observatory told Full Fact that the Telegraph’s analysis was “one more estimate of a number that’s incredibly hard to pin down” and that it was unable to say how accurate its figures were.

It added: “They may be plausible, if towards the upper end of the estimates we’ve seen over the years… yet at the same time much lower estimates may be equally plausible.

“Because of a lack of high quality data, any estimate of this population needs to make a number of relatively arbitrary assumptions to get to a final figure, and different assumptions can mean very different end results.”

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 use different definitions, and so should be treated with caution. The Migration Observatory has written more about these particular studies here. It’s worth noting it says in relation to the Pew figures that there are “significant questions about the accuracy of the estimates”, in particular due to its decision to exclude legal migrants with indefinite leave to remain from its estimate of the UK’s legally resident non-EU population.

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.”