Conservatives charged Blair with covering up the figure when controversy about the issue raged only a few weeks ago. Then, Blair said it was impossible to put a figure on the number of illegal immigrants - failed asylum seekers who have stayed on, visitors who have overstayed and clandestine entrants. When an independent academic, John Salt, of the Migration Research Unit at University College, London, suggested before polling day the true figure could be about 500,000, he was shot down by ministers. One called this ``wildly inaccurate'' and denied there were any official estimates. But now it is clear that Salt was helping the Home Office in its research and had contributed to earlier drafts of Thursday's report, which estimates the unauthorized migrant population at between 310,000 and 570,000.

The Home Office said the central estimate for the number of illegal immigrants was 430,000, or 0.7 percent of the total population. The figure does not include up to 772,000 asylum seekers whose applications are being processed or who have launched appeals.

Although the final report was completed only last week, reports of Salt's findings were circulating in April before the election was announced.

Yet during the campaign, when Blair was asked repeatedly on a BBC news program to give an estimate for illegal immigration, he said this was not possible. ``I can't be sure of the numbers of people who are illegals in this country, for the same reason that the previous government couldn't,'' he said. ``You cannot determine specifically how many people are here illegally.''

But while previously there were no official estimates, the Home Office had been working on one for almost a year at Blair's behest.

The Conservative opposition said he and other ministers must have had a general idea of the findings. Figures began circulating in Whitehall last autumn yet the government continued to deny any knowledge of the scale of the problem right up to election day, May 5.

A Downing Street spokesman said Blair did not know the results of the Home Office study at the time.

THE DAILY TELEGRAPH