The Home Office study, based on a formula developed in the United States, put the number of illegal migrants at between 310,000 and 570,000. It produced a "central estimate" of 430,000 people living illegally in Britain – amounting to 0.7 per cent of the population.
Before the election, Mr Blair denied there was any estimate of the number of people living illegally in Britain. The Prime Minister was asked about the issue 20 times on the BBC's Newsnight and insisted the information did not exist. Also this year, ministers were accused of covering up research sponsored by the Home Office suggesting that the number of illegal migrants could be between 450,000 and 500,000.
Tony McNulty, the Immigration minister, said yesterday: "This is only an estimate ... It is a useful contribution to the debate and it underlines the need for a robust ID card scheme which will, among other benefits, help tackle illegal working and immigration."
Mark Oaten, the Liberal Democrat home affairs spokesman, said: "The Government must grasp the nettle and look seriously at an amnesty for long-term illegal residents in exchange for much tougher border controls in the future. The resources don't exist to deport half a million people, and many of these individuals will have been here for years and have children."
The Government's controversial ID card legislation cleared its first hurdle in the Commons this week, but still faces entrenched opposition from Labour backbenchers and the House of Lords.
Downing Street denied that Mr Blair knew about the figures when he was interviewed by Jeremy Paxman on Newsnight, insisting that the latest research was only completed last month. The Home Office said earlier attempts to produce a figure for illegal immigrants were based on unreliable methods.
But Patrick Mercer, the shadow Home Office minister, told the BBC: "I suggest strongly that the Prime Minister has misled us for political gain in the lead-up to an election. It does just strike me as a particularly cynical move that once the election is out of the way, once the Government is safely ensconced with a reduced but nonetheless respectable majority, then they cough to these sorts of numbers at this stage."
He added: "If you were in government and trying to control things, you wouldn't want these sorts of figures coming out when the big issue of immigration and asylum-seeking ... was coming up as a political issue before the election."
Sir Andrew Green, chairman of the anti-immigration group Migrationwatch, said: "Having denied it vigorously before the election, the Government now admit that the number of illegal immigrants is approaching half a million. Even this looks like a considerable underestimate, with little allowance made for illegal entrants and those who overstay their visas."