London Home to Majority of UK’s Estimated Illegal Migrant Population

New analysis highlights the challenges of estimating undocumented populations and raises concerns about outdated data and policy gaps.

Studies estimate there are between 800,000 and 1.2 million undocumented migrants in the UK, with London hosting around 60% of this population.
A 2020 Greater London Authority report estimated 397,000 undocumented migrants in London, while a more recent Thames Water study suggests the number could be as high as 585,000.
The majority of undocumented migrants are believed to have entered the UK legally but overstayed visas, with visitor, student, and work visas being the most common categories.
Experts caution that all estimates of irregular migrant populations are highly uncertain due to limited data and methodological challenges, with no official government estimates since 2005.
Critics argue that the lack of reliable data complicates public service planning and enforcement, while debates continue over policy solutions such as deportations and potential amnesties.