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Irregular Migration in the UK: Definitions, Pathways and Scale

11 Jul 2011

This briefing discusses the complexities of defining and measuring irregular migration and what is known about its scale in the UK.

This briefing will be updated when new data becomes available

  1. Key Points
    • Irregular migration is by definition not recorded and eludes statistical coverage. The complexity and ambiguity of the terms ‘irregular migration’ and ‘irregular migrants’ further contribute to making precise measurement unfeasible.
    • The law defines immigration status in a binary way as either legal or illegal, but in practice irregular immigration status can involve a wide spectrum of violations of immigration and other laws.
    • There are many pathways to irregular migration status. The most frequent pathway to irregularity in the UK is likely to be ‘visa overstaying’.
    • Estimates about the size of the irregular migrant population in the UK are limited. The most recent estimate suggests a range of 417,000 to 863,000 irregular migrants (central estimate 618,000) at the end of 2007.
    • The estimated number of irregular migrants in the UK is among the highest in the EU.
  1. Understanding the Evidence

    The term ‘irregular migration’ typically refers to the cross-border flow of people who enter a country without that country’s legal permission to do so. In contrast, the term ‘irregular migrants’ typically refers to the stock of migrants in a country who are not entitled to reside there, either because they have never had a legal residence permit or because they have overstayed their time-limited permit. As explained below, it can also refer to migrants who are legally resident but breaching the conditions attached to their immigration status. Whether and to what extent legally resident migrants who are violating restrictions of their residence permit should be included in the population of irregular migrants is a highly contested question. Debates about what and who should be included under irregular migration and irregular migrants are further complicated by disagreements about the terminology that should be used to describe the phenomenon. One can find a plethora of terms in academia, policy and media contexts. These include, among others, clandestine, illegal, unauthorised, unlawful, undocumented, aliens without residence status, illegalised people, non-compliant, sans papier (without documents) as well as irregular. In addition to – and partly because of – the complexities of definitions, data on irregular migration and migrants are very limited. Irregular migration is by definition not recorded and eludes statistical coverage. Therefore, precise measurement is unfeasible.

The law defines immigration status in a binary way as either legal or illegal, but in practice irregular migration status can involve a wide spectrum of violations of immigration and other laws

Various types of violations of UK immigration regulations can lead to irregular migration status. “Illegal entry” can involve clandestine border crossing (i.e. evading immigration controls) as well as overt entry through ‘means of deception’. The latter is a more complex matter and involves a wide variety of practices that range from forged documents to deception about the ‘purpose of stay’. This refers to actions that directly concern the migrants themselves, but also third parties, which help or arrange the ‘deception’ of UK authorities.

Irregular immigration status can also arise from legal entry and illegal overstaying of a time limited visa. This could involve overstaying a short-term tourist visa or a longer term (but still time-limited) residence visa (e.g. a student or work visa).

A third pathway into irregularity, which is often clearly defined in legal terms but in practice much more contested, arises from the violation of restrictions attached to a legal residence permit. Time-limited residence permits can include various types of restrictions relating, for example, to employment, access to welfare benefits and family reunion. Employment restrictions can involve limitations on the number of weekly hours of paid work allowed (e.g. non-EU students are allowed a maximum of 20 hours during term time). In some cases it is illegal for migrants to change employers without permission. In law, any breach of conditions attached to the visa and original ‘purpose of stay’ can lead to irregular migration status and, therefore, deportation.

Because of the complexity of immigration and employment laws, and the wide range of possible restrictions attached to different types of immigration status, research has identified various grey zones of irregularity which blur the line between legal/illegal (regular/irregular). Düvell (2008) refers to a scale of regularity and irregularity while the threshold between the two is sometimes indefinable. This mixture of regular and irregular aspects can be manifold. For instance, migrants might have regular residence status but work without permission; or migrants may work with a permit though on another job than the one registered with the authorities; or may work longer hours than stipulated by the work permit (i.e. breach of condition). The terms ‘semi-legality’ (Düvell 2006), ‘formal informality’ (Erdemir and Vasta 2007) or ‘semi-compliance’ (Ruhs and Anderson 2010) describe this ambivalence.

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The UK’s population of irregular migrants is likely to be dominated by ‘visa over-stayers’ rather than ‘illegal entrants’

The consensus among researchers is that the majority of irregular migrants in the UK are likely to be visa over-stayers, i.e. migrants who entered the UK legally but overstayed their residence permit, rather than illegal entrants (Düvell 2009). There is also likely to be a significant number of migrants who are legally resident but working in violation of the employment restrictions attached to their immigration status (Ruhs and Anderson 2010). Other changes to the irregular migrant population, such as status-related flows (i.e. migrants moving from legal into an irregular status and vice versa) have only been researched very recently, but may contribute significantly to the quantitative composition of the irregular migrant population in the UK.

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The estimated stock of irregular migrants at the end of 2007 ranged between 417,000 and 863,000

There are only two formal studies that estimate the number of irregular migrants in the UK. Woodbridge (2005) estimated that the number of irregular migrants was between 310,000 and 570,000 (central estimate of 430,000) in 2001. This study used the 2001 UK Census and a ‘residual method’. This method simply deducts an estimate of the legally residing foreign-born population from the total number of foreign-born recorded in the UK Census. The difference (or the ‘residual’) is an estimate of the number of irregular migrants present at a specific point in time.

Gordon et al. (2009) used the same methodology but updated the estimates (including, for example, adjustments for formerly irregular migrants regularised by the accession of the A8 countries to the EU) and added an estimate of UK-born children of irregular migrants (a total of 85,000). Their estimates suggest a population of irregular migrants and their UK-born children of between 417,000 to 863,000 (central estimate 618,000) at the end of 2007. The study also estimates that about two thirds (central estimate of 442,000) of irregular migrants lived in London at the end of 2007 and that the number of refused asylum seekers in London in irregular status increased by around 131,000 since 2001.

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The estimated number of irregular migrants in the UK is among the highest in the EU

It has been estimated that approximately 10 to 15% of the world’s 214 million international migrants are in an irregular situation (IOM 2010).In the US, the current estimate is 10.8 million irregular migrants (January 2009), which implies a decrease of 0.8 million persons within two years (the estimate was 11.6 million in January 2008). However, between 2000 and 2009, the estimated irregular migrant population in the US grew by 27% (Hoefer et al. 2010).

There were an estimated 1.9 to 3.8 million irregular migrants in the EU in 2008 (European Commission 2010, Kovacheva and Vogel 2009). According to the Clandestino project (Clandestino n.d.), the estimated number of irregular migrants in the UK (417,000 – 863,000 estimated stock of irregular migrants in 2008 – adjusted estimate, for more details consult Clandestino project) is high in comparison to other European countries such as France (178,000 – 400,000), Germany (196,000 – 457,000), Italy (279,000 – 461,000) or Spain (280,000 – 354,000). The UK is one of the countries in the EU for which the estimates are of medium quality (as regards the estimates’ data and methodological rigorousness), as for other countries estimates are often of lower quality (Kovacheva and Vogel 2009). While the quality of estimates varies across countries and, therefore, it is not possible to assume that estimates are perfectly comparable, the estimates for the UK tend to be higher than for other EU countries.

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Evidence gaps and limitations

Given the data limitations, the ‘residual method’ has been the preferred approach used to estimate the number of irregular migrants in the UK. The residual method has important limitations. Above all, it may incorporate an unknown ‘residual of the residual’, i.e. an unknown number of ‘falsely’ recorded/unrecorded persons. For example, some irregular migrants may have returned completed Census forms, since the UK Census form states “to be completed by all people” and refusal may result in a fine. Second, enumerators were encouraged to identify the number of people at an address if occupants or neighbours provided them with any information. Therefore, in some cases incomplete or highly inaccurate data will have been included on forms. Third, Census data were adjusted for underenumeration, i.e. in some areas, mainly inner city areas, non-response from the population as a whole was relatively high and large adjustments were made. These areas are especially likely to host irregular migrant populations. Necessary statistical adjustments may have over or underestimated some local populations. UK demographers, including David Coleman and John Salt, have long called for better data, specifically a ‘Population Register’, to improve all aspects of population statistics in the UK, including the measurement of the stock of irregular migrants (Coleman 2008; House of Lords 2008).

In addition to significant gaps in our understanding of the size of the irregular migration population in the UK, we also have very little systematic information about their composition (e.g. gender, nationality, age structure, length of stay, etc) and how it differs from the legally resident migrant population, where irregular migrants work and their impact on the labour market and public services. There is also limited analysis of how precisely illegal status affects the labour market and social outcomes of migrants themselves.


  • Clandestino Project website, accessed 21/03/2011, and
  • Coleman, D. “Time for a Fresh Start – the Case for a UK Population Register.” Memorandum to the House of Commons Treasury Committee enquiry on Counting the Population. In Counting the Population, Written Evidence Volume II, 184-197. House of Commons Treasury Committee, London, 2008.
  • Düvell, F. “Pathways into Irregularity: The Social Construction of Irregular Migration.” Comparative Policy Brief, CLANDESTINO Research Project, Centre on Migration, Policy and Society, University of Oxford, 2009.
  • Düvell, F. “Clandestine Migration in Europe.” Social Science Information 47, no. 4 (2008): 479-497.
  • Düvell, F. “Irregular Migration: a Global, Historical and Economic Perspective.” In Illegal immigration in Europe: Beyond Control?, edited by Franck Düvell. Houndmills: Palgrave/MacMillan, 2006.
  • Erdemir, A., and E. Vasta. “Differentiating Irregularity and Solidarity: Turkish Immigrants at Work in London.” Working paper 07-42, Centre on Migration, Policy and Society, Oxford, 2007.
  • European Commission. “First Annual Report on Immigration and Asylum (2009).” Report from the Commission to the European Parliament and the Council, European Commission, Brussels, 2010. COM(2010)214.
  • Gordon I., K. Scanlon, T. Travers, and C. Whitehead. “Economic Impact on London and the UK of an Earned Regularisation of Irregular Migrants in the UK.” GLA Economics, Greater London Authority, London, 2009.
  • Hoefer, M., N. Rytina, and B.C. Baker. “Estimates of the Unauthorized Immigrant Population Residing in the United States.” Office of Immigration Statistics, Department of Homeland Security, Washington DC, 2010.
  • Select Committee on Economic Affairs. “The Economic Impact of Immigration.” House of Lords, London, 2008.
  • International Organization for Migration. “World Migration Report 2010.” IOM, Geneva, 2010.
  • Ruhs, M. and B. Anderson. “Semi-Compliance and Illegality in Migrant Labour Markets: An Analysis of Migrants, Employers and the State in the UK.” Population, Space and Place 16, no. 3 (2010): 195-211.
  • Kovacheva, V. and D. Vogel. “The Size of the Irregular Foreign Resident Population in the European Union in 2002, 2005 and 2008: Aggregated Estimates.” Working Paper No.4/2009, Database on Irregular Migration, Hamburg Institute of International Economics, Hamburg, 2009.
  • Vollmer, B. “Irregular Migration in the UK – Dichotomised Discourses and Changing Landscapes.” In Irregular Migration in Europe: Myths and Realities, edited by A. Triandafyllidou. Aldershot: Ashgate, 2010.
  • Woodbridge, J. “Sizing the Unauthorised (Illegal) Migrant Population in the United Kingdom in 2001.” Online Report 29/05, Home Office, London, 2005.


Dr Bastian Vollmer

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