As many as one in every 100 people in the UK is an illegal immigrant, according to a new government estimate.

For the first time, the Home Office today issued figures estimating the size of the unauthorised migrant population, which it put at between 310,000 and 570,000. That figure does not include asylum-seekers whose applications are being processed, or who are appealing against a refusal, a group put at between 716,000 and 772,000.

A Home Office research document team used data gathered in the 2001 census and American methodology to reach a "central estimate" of 430,000 illegal immigrants, or 0.7 per cent of the UK population. That includes those who have overstayed their visas and failed asylum seekers who have not left the UK.

Tony McNulty, the Immigration Minister, said: "This is only an estimate and should not be seen as a definite figure. No government has ever been able to produce an accurate figure for the number of people who may be in the country illegally - by its very nature, it is impossible to quantify accurately, and that remains the case."

Illegal immigration was a key battleground during the last election campaign, when Tony Blair insisted that it was simply impossible to say how many illegal immigrants were in the country and that it was "pointless to speculate".

The fact that the figures suddenly appeared less than two months later led today to accusations of political cynicism.

Patrick Mercer, the Conservative Home Affairs spokesman, told the BBC Radio's World at One programme: "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."

Mr Mercer, who thought that the figures appeared "slightly low", 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 and the rest of it was coming up as a political issue before the election."

Referring to the figures, Mr McNulty said the central estimate of 430,000 underlined the need for a "robust ID card scheme which will, among other benefits, help tackle illegal working and immigration".

But Richard Ford, Home Correspondent of The Times, thought that was an argument that could well backfire, if ID cards ever got off the ground.

He said: "We now know that there are an estimated 430,000 people in the country illegally, but we don't know their identity. If there were ID cards you would know who they are and that then raises the question: what do you do with them? Do you have an amnesty or do you remove them?

"We're removing people at the rate of 20,000 a year so you can see how many years it would take."

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