How many people in UK are actually illegal migrants?
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Executive summary
Estimates of the number of people in the UK without legal immigration status vary widely; reputable recent analyses place the range at roughly 700,000–900,000 after corrections to an earlier Pew figure, while government agencies say it is not possible to produce a definitive count [1]. Other local and advocacy studies have produced much higher single-city or local estimates (for example a Thames Water–commissioned study suggesting up to 585,000 undocumented residents in London), highlighting how methods and definitions drive large differences in results [2].
1. Why the numbers swing so widely — measurement, definitions and secrecy
There is no single administrative register of people “without permission” in the UK; the Home Office explicitly says it “is not possible to know the exact number of people currently resident in the UK without permission” and does not publish an official illegal‑population total [1]. Different studies define “unauthorised”, “illegal” or “irregular” in inconsistent ways — counting visa overstayers, those who entered clandestinely, failed asylum‑seekers, and/or their UK‑born children produces very different totals. The Migration Observatory at Oxford stresses that the population is “largely unknown” because people move between regular and irregular status and avoid detection [1].
2. The revised Pew range: 700,000–900,000 and why it changed
A widely circulated Pew Research Centre estimate that once suggested 800,000–1.2 million unauthorised migrants in the UK was formally revised. The retraction and correction reduced the plausible range to about 700,000–900,000 after analysts identified methodological errors — notably the original use of lawfully resident counts that excluded people with Indefinite Leave to Remain, which inflated the unauthorised estimate [1]. The Migration Observatory documents that correction and flags the inherent uncertainty in all such estimates [1].
3. Local studies and headline outliers: why London numbers can look enormous
Local studies often produce much higher figures because they focus on hidden or transient populations and use different proxy methods. A January 2025 Thames Water‑commissioned study suggested London could be home to as many as 585,000 people lacking legal status — roughly one in twelve Londoners — but that is a local estimate based on water‑use proxies and is not directly comparable to national surveys or Home Office administrative counts [2]. Such figures feed headlines but are neither national estimates nor consensus numbers [2].
4. What official UK statistics do and do not count
Home Office and ONS data capture many migrants — for example those who entered on visas, visitors, and most who claim asylum — but explicitly do not enumerate an “illegal population” as a separate, definitive figure [3] [4]. The ONS cautions that because of measurement limits “it is impossible to quantify accurately the number of people who are in the country illegally”, though some unauthorised people are included in other datasets [5]. The government’s immigration statistics focus on flows, small‑boat arrivals, detentions and enforced returns rather than an overall unauthorised population count [3] [6].
5. Small boats, flows and why arrivals do not equal resident counts
Public debate often focuses on Channel crossings: since 2018 hundreds of thousands have attempted irregular sea crossings according to various trackers and government releases, but those arrival counts are flow figures — not the stock of all unauthorised residents [7] [8] [9]. The Home Office notes that small‑boat arrivals accounted for roughly one‑third of asylum applicants in a recent year, underscoring that irregular arrivals are a substantial but not exclusive component of irregular migration [3].
6. Political uses, claims and fact‑checking
The 1.2‑million headline has been repeatedly invoked in political rhetoric; fact‑checkers and analysts (Full Fact, Migration Observatory) caution it is out of date and methodologically unreliable because it comes from the older Pew approach that was corrected in 2025 [10] [1]. Different organisations (think tanks, advocacy groups, local authorities) have incentives to emphasise higher or lower numbers depending on policy goals; the methodological diversity means political claims should be checked against source methods [2] [10].
7. What this means for policy and public understanding
Policymakers face choices under high uncertainty. Because no agreed national “stock” figure exists — and because estimates depend on definitions and data sources — policy debates framed around a single absolute number risk misleading the public [1] [5]. Available sources do not mention a definitive, government‑endorsed count of all people without legal status in the UK; analysts must therefore work with ranges, local studies and flow data while being explicit about their assumptions [1] [3].
Limitations: this piece relies only on the supplied sources and does not attempt to adjudicate beyond their methods; competing estimates exist and hinge on differing definitions and measurement choices [1] [2].