London is home to as many as 585,000 illegal migrants, equivalent to one in 12 of the city's population, according to a previously confidential report commissioned by Thames Water.
The study, conducted by Edge Analytics and demography experts at Leeds University, aimed to quantify "hidden" users of water services.
The report estimates between 390,355 and 585,533 illegal migrants live in London, with a median figure of 487,944.
Most illegal migrants arrived in the UK on work, study or visitor visas before overstaying, the report suggests.
Reform UK chairman Zia Yusuf said he was "not the least bit surprised" by the figures, stating that "anyone who has walked or lived in London knows in their bones these figures are true."
He accused the Office of National Statistics of "gaslighting" the public, pointing to how legal migration figures were revised upward by 20 per cent.
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"The Tories allowed hundreds of thousands of largely men to cross our Channel illegally and what did they do? They put them up in phones and basically give them every incentive to come here," he added.
Yusuf warned the situation represents "not just a national emergency, it's a national security emergency."
He expressed particular concern about demographics, stating: "90 per cent of the people crossing the Channel are men."
"The number of military age males making that journey legally surpasses the number of available soldiers, both standing and territorial," he said.
"And most of them come from Afghanistan, Syria, Iran - countries with not the friendliest track records to this country," Yusuf added, calling the situation "incredibly and grossly unfair."
In total, there are estimated to be more than one million illegal migrants in the UK, with 60 per cent of those in the capital.
Reform UK deputy leader Richard Tice told The Telegraph: "One in 12 people in London are here illegally, probably working illegally using taxpayer-funded public infrastructure and services. It is totally unacceptable."
"It's another reason why we need to properly control our borders and welcome those who come here legally but thank those who are here illegally as we return them where they came from," he added.
A Home Office spokesman said: "This Government is strengthening global partnerships and rooting out the criminal gangs who profit from small boat crossings which threaten lives."
The spokesman added they had "removed 16,400 illegal migrants in just six months, the highest figure in half a decade, making it clear that those who arrive illegally will be returned."
The Home Office confirmed 1,000 migrants had crossed the Channel so far this year, following 38,816 in 2024, the second highest total on record.
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The study, conducted by Edge Analytics and demography experts at Leeds University, aimed to quantify "hidden" users of water services.
The report estimates between 390,355 and 585,533 illegal migrants live in London, with a median figure of 487,944.
Most illegal migrants arrived in the UK on work, study or visitor visas before overstaying, the report suggests.
Reform UK chairman Zia Yusuf said he was "not the least bit surprised" by the figures, stating that "anyone who has walked or lived in London knows in their bones these figures are true."
He accused the Office of National Statistics of "gaslighting" the public, pointing to how legal migration figures were revised upward by 20 per cent.
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"The Tories allowed hundreds of thousands of largely men to cross our Channel illegally and what did they do? They put them up in phones and basically give them every incentive to come here," he added.
Yusuf warned the situation represents "not just a national emergency, it's a national security emergency."
He expressed particular concern about demographics, stating: "90 per cent of the people crossing the Channel are men."
"The number of military age males making that journey legally surpasses the number of available soldiers, both standing and territorial," he said.
"And most of them come from Afghanistan, Syria, Iran - countries with not the friendliest track records to this country," Yusuf added, calling the situation "incredibly and grossly unfair."
In total, there are estimated to be more than one million illegal migrants in the UK, with 60 per cent of those in the capital.
Reform UK deputy leader Richard Tice told The Telegraph: "One in 12 people in London are here illegally, probably working illegally using taxpayer-funded public infrastructure and services. It is totally unacceptable."
"It's another reason why we need to properly control our borders and welcome those who come here legally but thank those who are here illegally as we return them where they came from," he added.
A Home Office spokesman said: "This Government is strengthening global partnerships and rooting out the criminal gangs who profit from small boat crossings which threaten lives."
The spokesman added they had "removed 16,400 illegal migrants in just six months, the highest figure in half a decade, making it clear that those who arrive illegally will be returned."
The Home Office confirmed 1,000 migrants had crossed the Channel so far this year, following 38,816 in 2024, the second highest total on record.
Find Out More...