As many as one in 12 people in London are illegal immigrants, a "deeply alarming" new report has revealed.
A previously confidential report found that almost 600,000 people live in the nation's capital without the right to be in Britain - but the Home Office do not provide any official figures on the scale of the problem.
The figure - some 585,000 in London - has come to light after a study for Thames Water was brought to light by The Telegraph through freedom of information-style laws for the environment.
The report estimates that there are more than one million illegal migrants in the UK as a whole - 60 per cent of which reside in the capital.
As many as one in 12 people in London are illegal immigrants, the report found
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Almost 600,000 people live in the nation's capital without the right to be in Britain
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While the party's deputy leader Richard Tice warned: "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."
The Thames Water report, carried out by Edge Analytics and Leeds University data analysts, had aimed to work out how many people truly used its services to enable the water firm to better meet demand.
It based its research on national estimates of illegal migrants from the Pew Research Center in the US, the London School of Economics, and Office for National Statistics data.
It then used National Insurance registrations for non-EU foreign nationals over a nine-year period to estimate the number of so-called "irregular" migrants in each London borough.
Reform's Richard Tice called the illegal migration situation 'totally unacceptable'
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For London, the study placed its estimate at a minimum of 390,355 illegal migrants and a maximum of 585,533 at its highest, with a median figure of 487,944.
The capital's estimated population sits at 7,044,667. As a result, one in 12 of the capital's population is an illegal migrant.
When other areas outside London covered by Thames Water are included, like Henley, Guildford, Reading, Swindon and Newbury, the range for the number of illegal migrants rises to between 415,568 to 623,351.
But in the face of this, the Home Office does not publish any full data on the total number of illegal migrants in the UK, while only since 2018 has it publicised figures on the number of migrants crossing the English Channel in small boats.
The Home Office, responding to The Telegraph's findings, hailed how it had 'removed 16,400 illegal migrants in just six months'
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A Thames Water spokesman said: "Water companies have a regulatory obligation to undertake a 'water balance', which includes understanding how much water our customers use on a per-person basis, and how it is distributed across our supply area.
"Analysis to estimate 'hidden and transient' populations is carried out by an independent firm of consultants, who draw from publicly available sources including census, surveys, and published academic research.
"Thames Water played no part in the writing of the report and the conclusions drawn are those of the independent firm that carried out the research."
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.
"We have also 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."