You asked

I can't see any statistical estimate of the number of illegal immigrants in the UK. Do you have any figures on this and if not what action are you taking to collect this missing data. Can you let me have any estimates you may have covering the past ten years.

We said

By its very nature it is impossible to quantify accurately the number of people who are in the country illegally. For this reason ONS does not produce estimates on the size of the illegal migrant population.

In June 2005, the Home Office published the outcome of an assessment of whether methods used in other countries to estimate the size of the illegal population could be applied to the UK. The outcome estimated that the total unauthorised migrant population living in the UK in 2001 was 430,000. Please see the following reports for more information:

  • 29/05 – Sizing the unauthorised (illegal) migrant population in the United Kingdom

  • 58/04 – Sizing the illegally resident population in the UK

A further report has been written by the London School of Economics, which estimates that in 2007 the number of ‘irregular’ migrants was 533,000.

  • Economic impact on the London and UK economy of an earned regularisation of irregular migrants to the UK

To summarise the findings of these reports, although organisations have attempted to use data sources to estimate the number of illegal immigrants resident in the UK, the methodology behind this work requires huge assumptions thus making the estimates largely uncertain.

Although ONS do not currently estimate the illegal immigrant resident population, in theory they could be captured within some data sources (for example the Annual Population Survey or the Census). However, these surveys only record individuals as a resident of the UK and does not seek to identify those residing illegally.

Please do not hesitate to contact ONS at if you require any further help on long-term international migration.

The new first time User Guide1 for migration statistics provides an overview of the definitions and calculations used for the ONS immigration and emigration figures.