Tony Blair has been accused by the Tories of "misleading the public" over the number of illegal immigrants living in Britain.
The Prime Minister sparked controversy last week by repeatedly insisting that no Government had ever had a precise figure.
But shadow Home Secretary David Davis said yesterday that Mr Blair had commissioned an investigation into the issue.
"We believe there is an estimate in the Government's hands of 500,000," he said.
Mr Blair repeatedly refused to put a figure on the number of illegal immigrants when pressed last Wednesday by the BBC's Jeremy Paxman.
Challenged on this, Mr Davis said: "The problem there was that the Prime Minister, yet again, misled the public.
"We now know - I have the document here - that last March, the Prime Minister actually commissioned an investigation into the number of illegal immigrants in the UK and all the routes they came in by."
Mr Davis was referring to a confidential memo, titled "Asylum Stocktake", drawnup by two of Mr Blair's most senior civil servants.