Reform UK claims about the number of people in the UK illegally

With the Reform UK party conference underway in Birmingham today, we’ve heard a couple of senior figures from the party make claims about the number of people who are in the UK illegally.

On BBC Breakfast [34.15], the head of Reform UK’s department of government efficiency Zia Yusuf claimed: “There are north of a million people in this country illegally, at least—the real number’s probably higher than that. There are north of around 600,000 men who are in this country illegally.”

And addressing the conference this afternoon, party chair David Bull said: “We believe there are over one million people in this country who do not have the right to remain.”

It’s not immediately clear what these estimates are based on—we’ve asked Reform UK, and will update this blog if we receive a response.

But as we’ve written before, we don’t know with any certainty how many people are in the UK without the right to be here. The University of Oxford’s Migration Observatory has warned that studies attempting to put a number on the UK’s unauthorised migrant population should be treated with caution due to large margins of error.

Neither the Home Office nor the Office for National Statistics (ONS) currently publish estimates of the UK’s total irregular migrant population, and 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.”

It’s possible that today’s Reform UK claims may have been based on a study published by the Pew Research Center think tank in 2019, which estimated there were between 800,000 and 1.2 million unauthorised migrants in the UK in 2017, 48% of whom it estimated were male. This would produce an upper estimate of around 576,000 men in the UK without the right to be here. Mr Yusuf has previously referred to a figure of 1.2 million when talking about the number of people who are “here in this country illegally”.

However, as we wrote last month, the Pew Research Center has since acknowledged its methodology was flawed, and subsequently revised its estimate to a lower range of 700,000-900,000.

In any event, these figures and indeed most estimates of the UK’s unauthorised population are several years old and so are not necessarily representative of the true number in 2025.