The prime minister has acknowledged that he did not know the extent of the continuing failures to deport foreign criminals when he rejected Charles Clarke's offer to resign.

Amid increasing doubts over the home secretary's position, Tony Blair on Wednesday struggled to defend his cabinet colleague.

And speaking at prime minister's questions, Conservative leader David Cameron said that Clarke had misled the public and should now be removed from his post.

The home secretary admitted to the BBC Radio 4 Today programme that he had offered to resign but said the prime minister turned down his offer.

The row focuses on the Home Office's failure to deport more than 1,000 foreign offenders, with officials revealing that 288 had been released since ministers were first told of the problem last summer.

Cameron said Clarke's suggestion that the number released since last summer had been "very, very few" had turned out to be "completely misleading".

"This home secretary has presided over systemic failure. He has failed to deal with it and last night he misled people about the scale of the problem," said the Tory chief.

"Isn't it clear that he cannot give the Home Office the leadership it so badly needs?"

Blair insisted: "No I don't agree with that."

"I do not accept that the home secretary did not act on this matter. He did act."

But Blair was forced to tell MPs that he "did not know the details of the figures that the home secretary gave until today".

That prompted Cameron to say: "The prime minister backs incompetent ministers even when he doesn't know the facts. That is what we have discovered."

The public would now feel that "enough is enough", he added.

The home secretary had told the BBC that he had told Blair before Christmas of the release of criminals who should have been considered for deportation.

Asked whether he had offered to resign, he said: "Yes I did. I told him I was prepared to resign if he thought it was right. He said he didn't think it was right."

Clarke, later facing angry MPs in the Commons, faced more Conservative calls to resign.

But he said he was best placed to put things right and insisted it was "absolutely untrue that this government neglects public safety".

The home secretary said he would take responsibility and "put things right".

Earlier in the Commons, Liberal Democrat leader Sir Menzies Campbell also questioned whether the prime minister felt "any sense of embarrassment in presiding over such incompetence".

Blair conceded that there had been "longstanding systemic failure" but insisted that there was now "a proper system in place".

But Sir Menzies asked: "How can the home secretary remain in office? How can the prime minister not ask for his resignation?"

Cameron also said that Tuesday's revelations about the Home Office's failure had highlighted a "massive problem".

And measures taken  after the government was told about the difficulties were "completely insufficient".

Blair expressed his "deep regret" that the system had been "seriously and fundamentally at fault".

But he sought to emphasise that a third of those would had been released without being considered for deportation had subsequently seen their cases reviewed.

The prime minister added that from last July there had been an increase in the staff and resources allocated to dealing with the problem.

"That has taken time to build up," acknowledged Blair, but "since April 1 the system is now working properly".

And offering his explanation, Clarke acknowledged that the National Audit Office warned the government about the issue last summer.

"The truth is, it is like the old story of the tanker turning around," he told the BBC.

"We have very large organisations and it takes time to get it right, and that's what we are doing."

Asked if he believed he would still be in his job at the end of the day, he replied: "I certainly hope so. I want to carry this through and get it sorted out, and I think that's what people would want me to do."

But former Tory leader Michael Howard also called on Clarke to resign.

He told Today: "I'm afraid that if this government has an ounce of honour left, Charles Clarke has to go."

Howard, a former home secretary, said there was "no excuse" for the oversight.