Which regions and cities in the UK have the highest estimated populations of undocumented migrants?
Executive summary
Estimates and multiple studies converge on one clear finding: London and its surrounding South East corridor host the largest share of the UK’s undocumented or unauthorised population, though precise counts are highly uncertain and range widely depending on methodology [1] [2]. National-level research and advocacy groups stress both the methodological limits of any single figure and the political stakes in how these populations are counted and presented [3] [4].
1. London: the undisputed epicentre of undocumented residence
London stands out in every authoritative source as the region most likely to contain the largest absolute numbers of undocumented migrants, reflecting that it contains roughly 40% of the UK’s foreign‑born population and about half of all foreign‑born residents in London and the South East combined [1] [5]. Independent and commissioned analyses amplify this: a Greater London Authority-funded update and other research have produced London‑focused estimates running into the hundreds of thousands — one 2017‑based update put possible figures in the hundreds of thousands and a Thames Water‑commissioned analysis suggested London could host as many as 585,000 “hidden” or transient users, a finding that underlines the scale of uncertainty but reaffirms London’s primacy [2].
2. The South East and commuter belt: the secondary concentration
Beyond the capital, the South East region absorbs a substantial share of migrants and is therefore commonly included alongside London when analysts describe hotspots for unauthorised residence: census and migration briefings show around 47% of foreign‑born residents lived in London and the South East in 2021/22, implying the South East is the next most important region for any residual estimate of undocumented populations [1]. National statistics compilations and parliamentary briefings underscore that migration is geographically concentrated — which means residual‑method estimates that subtract authorised migrants from total foreign‑born or non‑British populations will inevitably allocate large shares to these neighbouring regions [6] [7].
3. Other cities and regions with notable undocumented populations — evidence is thinner
Outside London and the South East, the evidence for city‑level rankings of undocumented residents is sparse and contested: Migration Observatory briefings and ONS materials show lower foreign‑born shares in regions such as the North East, Wales and Northern Ireland (single‑digit percentages), which implies smaller unauthorised populations there, but they do not produce reliable city‑by‑city undocumented counts [1] [5]. Academic studies using the residual method have produced national ranges and occasional local estimates, but when they do localise findings the clearest pattern is that large urban centres with higher overall migrant populations — other English cities in the West Midlands, Greater Manchester, and parts of the West and East Midlands — are more likely than rural areas to contain greater numbers of unauthorised residents, even if specific numbers remain uncertain [3] [2].
4. Why numbers vary so widely: methods, definitions and data gaps
Estimates of “undocumented” populations depend on definitions (citizenship vs country of birth), the residual method’s assumptions, and which cohorts are included (for example, UK‑born children of undocumented parents), producing widely varying results and wide confidence intervals; Migration Observatory analysis and comparative studies warn that the residual approach is considered the best available but still produces divergent estimates depending on inputs and timeframes [3] [8]. Home Office, ONS and parliamentary statistical releases deliver robust legal‑migration totals and flows but do not enumerate all unauthorised residents directly, meaning all regional breakdowns of undocumented populations are inferential rather than direct counts [7] [6].
5. Politics, advocacy and the hidden agenda in counting the undocumented
Advocacy groups and research bodies bring competing implicit agendas to the debate: organisations like JCWI highlight structural causes and call for regularisation pathways, framing high estimates as evidence for policy change [4], while some commissioned private studies — for example utilities‑based analyses — may emphasise “hidden” users in ways that attract public attention but are methodologically controversial [2]. Parliamentary and official sources focus on migration flows and system statistics [6] [7], often sidestepping definitive unauthorised counts; readers should therefore treat regional claims as indicative rather than definitive and recognise that different actors use the same data to support divergent policy prescriptions [3] [8].