As many as one in 12 people in London are illegal immigrants, a shock report claims.
The previously confidential study commissioned by Thames Water found that the capital's 'irregular' population could be more than half a million.
It was intended to quantify the “hidden” users of the utility's services to better meet demand.
Researchers from Edge Analytics used academic estimates of illegal migrants at national level and national insurance records for non-EU foreigners over a nine-year period to estimate the numbers in each London borough.
The study produced a range of figures from 390,355 at the lowest level to 585,533 at the highest level, with a median of 487,944.
It suggested that most illegal migrants arrived in Britain on work, study or visitor visas and then overstayed.
The report obtained by The Telegraph claimed that there are estimated to be more than a million illegal migrants in Britain, with 60 percent of them in the English capital.
Many illegal migrants arrive in Britain on work, study or visitor visas and then go over the limit.
Despite poor visibility, two rubber boats with 190 people on board were spotted by Border Force vessels on Tuesday
It took just 21 days this year for the number of crossings to reach 1,000 – the fastest time ever recorded
The Home Office does not publish comprehensive data on the number of illegal migrants in Britain.
It has only published figures on the number of illegal Channel migrants reaching Britain since 2018.
Shadow Home Secretary Chris Philp said: 'It is completely unacceptable that there are so many illegal immigrants in Britain.
“The law needs to be looked at so that false human rights, modern slavery and asylum claims cannot be used to delay or prevent the removal of illegal immigrants.”
Richard Tice, the vice-chairman of Reform UK, told The Telegraph: 'One in 12 people in London are here illegally and are likely to be working illegally using taxpayer-funded public infrastructure and services. It's totally unacceptable.
“It's another reason why we need to properly control our borders.”
Yesterday it was reported that more than 1,000 migrants have crossed the English Channel in small boats so far this year, a new record that exposes Sir Keir Starmer's struggles to deal with the crisis.
Two dinghies carrying 129 people were spotted by Border Force vessels on Tuesday despite poor visibility, bringing the provisional total for 2025 so far to 1,019 out of 18 boats.
This means that this year it took just 21 days for the number of crossings to reach 1,000 – the fastest time ever recorded.
Inflatable dinghies and outboard motors stored at a Port Authority yard in Dover
Those on board were taken to the port of Dover, Kent after being taken aboard Border Force catamarans halfway across the Channel.
The latest figures add pressure on the Prime Minister as he struggles to get to grips with the issue.
In November, Sir Keir said the government was willing to pay governments in Vietnam, Kurdistan and Turkey to help stem the flow of illegal migrants into Britain.
The Prime Minister said he was willing to look at 'anything' that would help slow the wave of migrants crossing the Channel illegally.
He declined to comment immediately on reports that the government is in advanced talks with governments in Kurdistan and Vietnam over deals where Britain would pay them to help stop migrants heading to Britain.
But he indicated he was not against the principle of paying third countries to help tackle the migrant crisis in the Channel.
“Anything else we can do to keep people from leaving is the right thing,” he said.
However, the Tories said the government had lost control of Britain's borders.
Last year, 36,816 people successfully crossed the busy strait in the deadliest year on record for small boat migrants.
A group of people believed to be migrants are taken to Dover, Kent, on January 21
Shadow Home Secretary Chris Philp, pictured, said the government had lost control of Britain's borders
A Home Office spokesperson said: 'This government is strengthening global partnerships and rooting out the criminal gangs that profit from small boat crossings that threaten lives.
“We have also removed 16,400 illegal migrants in just six months, the highest number in half a decade, making it clear that those who arrive illegally will be sent back.”
Home Office sources told The Telegraph that the underlying data was outdated and there was no officially recognized way to measure how many illegal migrants there are in Britain.
A major rescue operation was launched on Monday to save 84 migrants crammed into a rubber boat after it became stranded on a sandbank as they tried to cross the English Channel.
The French coast guard was alerted after witnesses on a beach saw the overloaded inflatable boat trying to cross to Britain.
A nearby French customs ship was sent to the area to monitor the dinghy at Walde, near Calais.
After arriving, the crew reported that the dinghy was stranded on a sandbank, but they could not get close enough to help because the water was too shallow.
The stranded migrants also made an emergency call to French authorities.
A naval aviation rescue helicopter was called in, as well as another boat to assist with the rescue as the tide began to rise.
People, believed to be migrants, disembark from a British Border Police ship as they arrive at the port of Dover on January 15
Cash-only Brits, hairdressers or car washes are 'fueling the problem' of illegal migration and could create more victims of slavery, a top Home Office official has warned
A total of 22 people isolated by the tide were removed from the sandbank and transferred to the customs boat.
The remaining 62 on board the dinghy were able to continue their attempts to reach Britain, but they too sent out a second distress signal.
The rescue tug Abeille Normandie, chartered by the French Navy, went to assist and picked up the 62 people still on board the boat before dropping them off in the French port of Boulogne-sur-Mer, where they were all met by the rescue effort . onshore services.
A Thames Water spokesperson said: 'The analysis to estimate 'hidden and transient' populations is carried out by an independent consultancy, drawing on publicly available sources including censuses, surveys and published academic research. Thames Water had no role in the writing of the report and the conclusions drawn are those of the independent agency that conducted the investigation.”
Hundreds of people were arrested last month as the government cracked down on illegal working in car washes, nail salons and supermarkets in London.
Nearly 1,000 enforcement visits were carried out in the capital by the Home Office's Immigration Enforcement team between July and November.
Nearly 1,000 enforcement visits were carried out in the capital by the Home Office's Immigration Enforcement team between July and November. Pictured: Stock photo of immigration enforcement officers
In that time, 996 visits resulted in 770 arrests and 462 properties received a civil penalty referral. If employers are found guilty, they could be fined up to £60,000 per employee.
During a recent visit to a hotel in Kensington, six agency employees were arrested, five on suspicion of working illegally and one found to have overstayed his visa.
Minister for Border Security and Asylum Dame Angela Eagle MP said: 'Dismantling criminal gangs and restoring order to our asylum system is an important part of our Plan for Change.
“That's exactly why we're tackling illegal working to prevent this abuse of our immigration system and protect those who are given a false promise about their ability to live and work here.
“Since the election, arrests and enforcement visits have deliberately increased, demonstrating that we will not tolerate any hiding place for law enforcement officers.”
The raids targeted car washes, nail bars, supermarkets and construction sites suspected of hiring illegal workers and subjecting them to squalid conditions and illegal working hours below the minimum wage.
It comes after a top Home Office official warned that cash-only Britons, nail bars, hairdressers or car washes could be “fueling the problem” of illegal migration and potentially creating more victims of slavery.
