London is home to as many as 585,000 illegal migrants, a report has claimed.
The estimate - the equivalent to one in 12 of the city’s population - comes in a study commissioned by Thames Water first reported by the Telegraph.
According to the data between 390,355 and 585,533 people in London would be considered illegal migrants as per the study.
The report suggests most illegal migrants arrived in the UK on work, study or visitor visas and then overstayed.
The report was accessed by the Telegraph through freedom of information-style requests.
The Home Office does not release data regarding illegal migrant numbers in the UK, and exact data on this topic is often difficult to determine.
The study suggested that a median range of around 1 million unauthorised migrants could be living in the UK, with 60% of the UK’s illegal migrant population based in the capital.
Migration experts suggested that the figures could be an underestimate as some of the underlying data dates from 2017, before the surge in migrants crossing the Channel.
The research was initially designed to identify “hidden” and “transient” users to better meet the demands of customers, looking at National Insurance registrations of non-EU nationals to identify “irregular” migrants across London boroughs.
There are a number of ways that people in the UK could be considered illegal migrants, such as overstaying work and student visas, entering illegally, overstaying after asylum terms, or being born to illegal migrant parents in the UK.
According to the Joint Council for the Welfare of Immigrants, the majority of illegal migrants in the UK initially came here legally before their situation changed.
Although there are no concrete figures, most undocumented migrants are thought to be from Asia, sub-Saharan Africa and the Americas.
The latest findings came amid reports that the number of migrants crossing the Channel so far this year has topped 1,000, a record number.
On Tuesday, the Home Office revealed that 129 migrants crossed the Channel with many more arriving on Wednesday.
One migrant, a Syrian man in his 20s, was found dead earlier this year according to French authorities, “probably” crushed to death on a flimsy dinghy - marking the first documented migrant death in the Channel of 2025.
In 2024, UK Prime Minister Sir Keir Starmer vowed that his government would "reduce immigration - legal and illegal," however the party has come under fire for not doing enough.
When pressed by reporters, Mr Starmer said cutting immigration "will only be done with a serious plan," adding that going after people smuggling gangs is the most impactful way to combat illegal immigration.
