How Many Illegal Immigrants in UK – Latest 2024 Estimates

No official census can count people who, by definition, seek to avoid detection. Yet the question of how many illegal immigrants are in the UK remains one of the most frequently asked—and most contested—in British public life. Understanding the numbers requires navigating conflicting estimates, data gaps, and fundamentally different ways of measuring a population that deliberately stays out of sight.

Government statistics capture arrivals through specific channels, such as small boat crossings and asylum applications, but these represent only the visible portion of irregular migration. The true scale of undocumented residence in the UK has been estimated by researchers using surveys, administrative data, and statistical modelling—with results that vary by hundreds of thousands depending on methodology.

The Home Office records enforcement actions and returns, providing one measure of the system’s response to unauthorized presence. Combined with independent research from the Migration Observatory and the Office for National Statistics, a clearer—though still incomplete—picture emerges of irregular migration’s scope and trajectory.

How Many Illegal Immigrants Are There in the UK?

The honest answer is that no one knows for certain. What exists instead are estimates, each based on different methods and assumptions.

Key Takeaways

• Estimates of the UK’s undocumented population range from 600,000 to over 1,200,000 depending on methodology used
• The Migration Observatory places the figure between 700,000 and 900,000, incorporating data from Greater London Authority analysis
• Around 85,000 refused asylum seekers from 166,000 refusals between 2010 and 2023 remain unrecorded as having departed the UK
• The asylum application backlog stood at approximately 97,000 cases by the end of September 2024, with 25,539 awaiting decisions on small boat-linked claims
• Hotel accommodation for asylum seekers housed 32,059 people according to data from August 2025
• Returns of those with no legal right to remain reached 31,500 in the year ending September 2024, the highest figure since 2017

Snapshot of Available Data

Why Is There No Official Count of Illegal Immigrants in the UK?

The Office for National Statistics has explicitly stated that it does not produce official estimates for net illegal migration. In a freedom of information response, the ONS directed queries to Home Office irregular migration statistics, acknowledging the fundamental measurement problem.

Several factors make accurate counting impossible through conventional means.

Data Collection Limitations

People without legal status typically avoid official interactions that would register their presence. Standard data collection methods—censuses, household surveys, administrative records—cannot capture individuals who have no incentive to identify themselves. Someone in the UK without authorization faces potential detention and removal, making voluntary disclosure virtually nonexistent.

Exit controls at UK borders remain incomplete. The government records arrivals through visas and border checks but lacks comprehensive departure tracking. This gap means researchers cannot reliably calculate how many people have overstayed their visas by comparing entry and exit data.

Methodological Variations

Different organizations arrive at vastly different estimates because they use different definitions, data sources, and assumptions. The Migration Observatory notes that unauthorized migration estimates carry significant uncertainty margins.

Pew Research initially estimated 800,000 to 1,200,000 undocumented migrants before revising the 2024 figure downward to 700,000 to 900,000 after methodology review. The Greater London Authority, using different assumptions, produced an estimate of 674,000 to 809,000 for the capital alone. These variations highlight how sensitive estimates are to underlying assumptions.

What Are the Main Entry Methods for Irregular Migrants?

Irregular entry to the UK occurs through several distinct pathways, each with varying degrees of visibility in official statistics.

Small Boat Crossings

Small boat crossings through the English Channel represent the most visible and politically prominent route of irregular entry. In the year ending March 2024, 31,079 people arrived on 625 boats—41% fewer vessels than the previous year, but with higher average occupancy per boat.

Afghan nationals comprised the largest group at 5,662 arrivals (19%), followed by Iranian (3,639, 12%) and Turkish (3,244, 11%) nationals. From January 2018 through March 2023, cumulative small boat arrivals reached 87,788, generating 80,448 linked asylum applications.

The government has pursued diplomatic and legislative responses, including the Border Security Bill, which proposes criminal offences for individuals who arrive in the UK without valid leave after crossing small boats.

Visa Overstaying

Visa overstaying—remaining in the UK after a visa has expired—likely constitutes the largest component of undocumented residence, though precise figures remain unavailable. Unlike small boat arrivals, overstayers enter legally and initially comply with immigration controls, making them harder to detect.

The Home Office collects limited data on inadequately documented air arrivals for the year ending March 2024. Iranian nationals accounted for 959 such detections (27%), with Afghan nationals at 350 (10%). Within the UK, recorded detections included Sudanese nationals (714, 21%) and Eritrean nationals (587, 17%).

Other Irregular Entry

Beyond small boats and visa overstays, irregular entry can occur through document fraud, using false identities at border controls, or entering through unmonitored routes such as ports with minimal security infrastructure.

How Does Illegal Immigration Fit into Broader UK Migration Trends?

Irregular migration operates alongside—and often gets conflated with—legal migration. Understanding the broader context helps separate what is known from what is speculated.

Net Migration Trends

Overall net migration to the UK has declined significantly from its peak. According to the Migration Observatory, net migration reached 906,000 in the year ending June 2023 before falling to 728,000 the following year—a 20% decrease. The 2024 full-year estimate stands at approximately 430,000.

Migration Watch describes current levels as “catastrophically high” compared to pre-Brexit norms of around 250,000, though the direction of travel shows improvement. These figures, however, cover only legal migration tracked through visa systems and do not include irregular flows.

Relationship Between Legal and Irregular Migration

Irregular migration exists somewhat separately from the legal net migration figures published by the ONS. Someone entering on a tourist visa and remaining indefinitely does not appear in departure statistics. A person crossing the Channel in a small boat and claiming asylum generates different data than someone who overstays a work visa. For the latest 2024 estimates on illegal immigrants in the UK, please see Dunham Massey National Trust fees. Dunham Massey National Trust fees

The asylum backlog of 97,000 cases creates extended periods of legal limbo. Individuals awaiting decisions on asylum claims have lawful status during the process but, if refused, may become undocumented if they do not leave.

Government Enforcement Response

Returns of those with no legal right to remain have increased substantially. The year ending September 2024 saw 31,500 total returns, the highest figure since 2017 and 19% above the 2013 baseline. By early December 2024, approximately 29,000 returns had been recorded—a 25% increase year-on-year.

Between July 5, 2024, and January 4, 2025, the Home Office conducted 34 charter flights and recorded 16,400 total returns, including 4,390 enforced returns. Foreign national offender returns increased by 23% to 2,580, while illegal working visits rose by 32% to 4,530, resulting in 3,190 arrests—a 29% increase.

Historical Timeline of UK Irregular Migration Estimates

Tracking how estimates have evolved reveals the challenges inherent in measuring hidden populations over time.

1. 2017: University College London publishes estimate of 745,000 undocumented migrants, widely cited as the most rigorous academic attempt to count the population
2. 2018: Windrush scandal reveals systematic failures in immigration enforcement, affecting individuals who had legitimate rights to remain
3. 2018–2022: Small boat crossings accelerate dramatically, with cumulative arrivals reaching 87,788 by March 2023
4. 2022: Record year for small boat arrivals at approximately 45,000
5. 2023: Peak net migration of 906,000 draws political attention to overall immigration levels
6. 2024: Small boat arrivals decline 31% to 31,079; net migration falls toward 430,000; Border Security Bill introduced

What Information Is Certain and What Remains Unclear?

Context and Broader Implications

The difficulty of counting undocumented migrants has direct policy consequences. Without reliable population estimates, evaluating the effectiveness of enforcement measures becomes challenging. Return statistics measure outputs—the number of people removed—but cannot determine whether the total undocumented population is growing, shrinking, or stable.

The Home Office irregular migration data provides the most comprehensive official source for detected arrivals, updated quarterly. Independent analysis from the Migration Observatory offers academic context and methodological transparency, acknowledging where uncertainty lies.

Political discourse frequently cites specific figures as definitive counts, but the reality involves ranges, estimates, and acknowledged gaps. Migration Watch cites Pew’s original 800,000 to 1,200,000 figure, while the Migration Observatory presents the revised 700,000 to 900,000 range. Neither claims precision.

Sources and Credibility

Understanding UK irregular migration requires consulting multiple authoritative sources, each with distinct strengths and limitations.

The unauthorized migrant population in the UK is uncertain, with different organizations producing estimates based on varying methodologies and assumptions.

— Migration Observatory, University of Oxford

Primary government sources include the Home Office’s quarterly irregular migration statistics and returns data, published on GOV.UK. The Office for National Statistics provides net migration data covering legal flows but explicitly excludes irregular migration from its estimates.

The Migration Observatory at Oxford University offers detailed academic analysis of irregular migration methods and challenges, including the estimated 85,000 refused asylum seekers who may have remained in the UK unrecorded.

Summary

No definitive answer exists to the question of how many illegal immigrants are in the UK. What can be said with confidence is that estimates span a wide range—from around 600,000 to over 1,200,000 depending on methodology—and that no official body claims to know the true figure. The government tracks arrivals through specific channels and measures enforcement outcomes, but the total undocumented population remains unknowable through current data collection methods.

Small boat crossings command political attention and generate reliable arrival data, but visa overstaying likely represents a larger undocumented category. Returns have increased substantially, reaching 31,500 in the year ending September 2024, suggesting enhanced enforcement capacity. Meanwhile, the asylum backlog creates ongoing uncertainty about the legal status of tens of thousands of individuals.

For those seeking to understand the scope of irregular migration, the most reliable approach involves consulting The Thin Blue Line reporting on enforcement activity alongside official statistics and independent academic analysis. Claims of precise undocumented population figures should be treated with appropriate skepticism given the fundamental measurement challenges involved.

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