John Reid today confirmed that the Government has no idea of the precise number of foreign nationals living illegally in Britain.

The Home Secretary blamed the Conservative administration of the 1990s for losing control of immigration as he said that, at best guess, there were around 400,000 failed asylum seekers, visa overstayers and clandestine entrants in the UK.

The Government's control over immigration was denounced as a mockery by MPs after officials told the all-party Home Affairs Select Committee that he "had not the faintest idea" of the number of people living in Britain illegally.

Dave Roberts, Director of Enforcement and Removals at the Immigration and Nationality Directorate (IND), revealed that the department was not actively pursuing individual overstayers to remove them because this is "not an effective strategy".

Asked three times how many people were not complying with reporting restrictions or had ignored instructions laid down by the immigration service, he replied: "I cannot answer that in the direct way you ask it."

MPs were astonished at the admissions which follow intense concern over the Government's failure to deport more than 1,000 of foreign criminals. David Davis, the Shadow Home Secretary, said that the latest revelations "beggared belief".

The failure of officials to provide any figures to the committee is deeply embarrassing, only days after the Prime Minister made removals a key objective for John Reid, the new Home Secretary. Mr Blair sent a letter to Mr Reid saying that he wanted additional progress made in tackling the stock of failed asylum-seekers and illegal immigrants.

Mr Reid today admitted it was “disturbing” that the figures were not available but he stressed that data on illegal immigrant numbers had not been collected for at least 15 years.

“This a disturbing fact but it is not a new fact by any means," he said.

The Home Secretary used less colourful language that his civil servant colleagues when questioned over the issue and expressed faith that a long-standing target to deport more failed asylum seekers each month than arrive would tackle the backlog.

"The truth of the matter is that we don't have a precise figure for this - not only do I not have it but none of my predecessors, way back to Michael Howard in the last government. And part of the reason is, of course, that we inherit a legacy of an unknown number from government to government as it were and therefore there are no precise numbers for this."

He told the BBC: "What we do know is that the number of false asylum seekers coming here is now being dealt with quicker than ever, that the number who are being removed from the country is greater than ever.

"I hope that the figures out soon will show that we have met our objective of removing more false asylum seekers in this country than are coming in."

At yesterday's committee hearing, exasperated MPs clashed repeatedly with Home Office officials when they were unable to give detailed answers to questions about efforts to curb illegal working and remove illegal immigrants from Britain.

Mr Davis said that there was 'no excuse' for not tracking illegal immigrants and moving them out of the country.

He said: "It beggars belief that along with our porous borders, problems with foreign criminals and our inability to deport a quarter of a million failed asylum seekers we also manage to hand out National Insurance numbers without checking up on a person's immigration status."

David Winnick, the deputy chairman of the committee, told the officials that their inability to provide figures "made a mockery of immigration control".

The committee was told by the chief economist at the Department for Work and Pensions that between 200,000 and 300,000 national insurance numbers were issued each year to foreigners and that immigration checks were carried out on only 2 per cent.

John Denham, the select committee chairman, wondered aloud what a worried constituent ordered to leave the country should be advised to do.

"I get the impression the correct answer for me to give would be 'not very much, they don't track individuals. I wouldn't worry about it'," he said.

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