An estimate of the number of illegal immigrants in the UK has not been produced for 15 years, the National Audit Office says.
An up-to-date estimate of the number of illegal immigrants in the United Kingdom has not been produced for 15 years, according to a report.
The National Audit Office (NAO) said the last estimate in 2005 suggested there were around 430,000 people in the country with no right to remain.
But independent research since has put the figure at more than one million, Whitehall's spending watchdog said.
The NAO's report into the Home Office's immigration enforcement directorate said the department had estimated demand for immigration enforcement activity.
This was put at between 240,000 and 320,000 cases a year.
But the report said there was no baseline given against which progress can be measured or to demonstrate whether demand is rising or falling.
The watchdog concluded that despite collecting information around its mission and objectives, the Home Office often cannot demonstrate whether its measures are working.