The information requested falls within the responsibility of the UK Statistics Authority. I have asked the authority to reply.
Letter from Karen Dunnell, dated April 2009:
As National Statistician, I have been asked to respond to your question concerning what estimate the Office for National Statistics has made of how many and what proportion of the working population are (a) legal and (b) illegal migrants (269154).
The Office for National Statistics collects data on the working population from the Annual Population Survey which covers residents of the UK. However this does not collect information on the legal status of workers.
On 30 June 2005 the Home Office published the outcome of the assessment of the applicability to the UK of the methods used by researchers and government agencies in other countries to estimate the size of the illegal population. A copy of the Research Development Statistics (RDS) On-line report 29/05—“Sizing the unauthorised (illegal) migrant population in the United Kingdom in 2001” can be found at:
http://rds.homeoffice.gov.uk/rds/pdfs05/rdsolr2905.pdf