This report reviews all relevant data sources on irregular migration (e.g. apprehended aliens at the border or in the inland, expulsion orders, people registered through health or other welfare schemes for undocumented immigrants, municipal registers, statistical estimates from national and European statistical services), assesses the validity of the different estimates given and where appropriate produces a new estimate for the year 2008 for the United Kingdom. It covers the period between 2000 and 2007 and the last year for which data or estimates were available when the study was finalised in 2009. This quantitative analysis is complemented by a critical review of qualitative studies and by interviews with key informants with a view to exploring the pathways into and out of undocumented status in the UK.
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