A new report shows that up to 585,000 people living in London might be illegal migrants. This number means about one in 12 people in the city could be living without legal immigration status, creating big challenges for the UK’s immigration system.
The study, commissioned by Thames Water and reported by the Telegraph, estimates that between 390,355 and 585,533 people in London are unauthorized residents. Many of them likely entered the UK legally on work, study, or visitor visas but stayed longer than allowed, becoming illegal migrants.
The UK Home Office does not provide official numbers on illegal migrants, making it hard to know the exact figures. This study uses older data, some of it from 2017, before the recent increase in migrants crossing the English Channel. Experts say the real number of illegal migrants could be even higher than the report suggests.
There are about 1 million illegal migrants in the UK, according to the report, and 60% of them live in London. This shows how London has become the main center for undocumented migrants in the country.
People become illegal migrants in different ways. Some overstay their visas, others enter the UK without proper documents, and some stay after their asylum claims are rejected. Children born in the UK to parents without legal status are also counted as illegal migrants. These cases show how complicated the problem is.
The report says illegal migrants in the UK come from many regions, including Asia, sub-Saharan Africa, and the Americas. This diversity shows that migration to the UK comes from all over the world.
Crossings over the English Channel remain a serious issue. On Tuesday, 129 migrants crossed the Channel in small boats, bringing the total number of arrivals in 2025 to more than 1,000. Sadly, the first migrant death of the year happened when a young Syrian man in his 20s died during the dangerous journey.
UK Prime Minister Sir Keir Starmer has promised to reduce both legal and illegal immigration. He says the government needs a strong plan to stop smugglers and prevent illegal crossings. But some critics believe the government is not doing enough to solve the problem.
The Thames Water study was created to help plan public services, like water supplies, by finding “hidden” and “transient” users. It used National Insurance registrations of non-EU nationals to estimate how many illegal migrants live in different parts of London. Even though this was the study’s main purpose, its findings have started a wider discussion about the scale of illegal migration in London and across the UK.
This report shows how hard it is to manage immigration in a global city like London. It also highlights the need for better data to help the government make informed decisions. As immigration remains a major issue in the UK, this report could influence future policies and public opinions.