What methods do UK authorities use to estimate the number of undocumented migrants?
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Executive summary
UK authorities and national statisticians do not have a single official count of people living in the UK without legal permission; instead, statisticians rely on a mix of administrative-data-based population models, the "residual" method that subtracts recorded lawful migrants from total migrant populations, targeted studies using expert elicitation (Delphi panels), and ad hoc research drawing on local administrative signals — each with important caveats and large uncertainty [1] [2] [3].
1. The residual method: subtracting the known from the total
A common approach used in academic and policy studies to estimate unauthorised migrants is the residual method, which takes an estimate of the total migrant population (by citizenship or country of birth) and subtracts the number of lawfully resident migrants to produce a residual assumed to be unauthorised; this is the technique behind high-profile estimates such as the Pew and some Greater London Authority (GLA) work, but it is sensitive to which base totals are used and who is counted as “authorised” [4] [2] [5].
2. Administrative-data population models and RAPID: a new backbone for migration totals
The Office for National Statistics (ONS) has been moving away from traveller surveys to administrative-data-driven population estimates — notably the Department for Work and Pensions’ Registration and Population Interaction Database (RAPID) and other administrative-based population estimates (ABPEs) — and these underpin the updated totals that feed residual calculations; ONS argues RAPID aligns more closely with UN definitions for long‑term migrants and allows new breakdowns by nationality and reason for migration [6] [7] [8].
3. The limits of exit checks, IPS and survey-based signals
Historically the International Passenger Survey (IPS) provided key arrival/departure signals, but IPS departures have been retired and exit checks have been paused or revised, leaving gaps that make tracking visa overstayers — a core component of the unauthorised population — difficult; ONS has used interim IPS data and assumptions while developing new methods, but admits provisional estimates rely on assumptions about length of stay and lagged travel observations [9] [10] [7].
4. Local studies, Delphi panels and indirect indicators
Beyond national modelling, estimates often rely on local research and expert judgement: Delphi panels (structured expert consensus) have been used at city and borough level, and city-level studies have combined housing, health, school and utility data (for example a Thames Water study flagged “hidden” users) to infer numbers — useful for local planning but hard to verify nationally and highly sensitive to definitions and data quality [3] [5].
5. How independent researchers and observatories treat uncertainty
Independent analysts such as the Migration Observatory and fact‑checking organisations stress that all existing estimates have large margins of error and methodological trade‑offs; they have highlighted problems in previous high-end estimates (for example Pew’s initial figures) where the exclusion or miscoding of lawful residence categories like indefinite leave to remain produced inflated residuals, leading to revisions and retractions [1] [4] [2].
6. Practical consequences: data gaps, political incentives and hidden agendas
Data gaps on visa overstayers and the discontinuation or revision of exit‑check statistics have practical effects on residual estimates, and political debates over immigration policy create incentives on all sides to emphasise high or low figures; while ONS and DWP emphasise methodological robustness in moving to RAPID, critics note that administrative sources can undercount transient or informal labour and that local studies commissioned by councils or utilities may be used to press for either tougher enforcement or greater service protections depending on the author’s agenda [8] [6] [5].
7. Bottom line: a layered, uncertain toolkit, not a single authoritative number
There is no single official UK figure for the undocumented population published by the Home Office or ONS; instead a layered toolkit is used — administrative population models (RAPID/ABPE), the residual method, targeted local studies and expert elicitation — and each method brings strengths (better coverage, transparency) and weaknesses (definition sensitivity, missing exit data, local verification problems), so all outputs should be read as indicative with wide uncertainty intervals rather than precise counts [7] [2] [3].