He branded ex-junior minister Tom Watson, the most senior person to quit, “disloyal, discourteous and wrong” for signing a letter urging him to go.

The resignations came as Mr Blair faces growing pressure to name a departure date or even quit now. Gordon Brown backers say assurances he will resign in May are “not enough”.

Mr Watson and the six parliamentary private secretaries (PPSs) – who are unpaid ministerial aides at the bottom rung of the government ladder – were among 17 normally loyal Labour MPs who signed a letter calling on Mr Blair to quit. The six were: Khalid Mahmood, Wayne David, Ian Lucas, Mark Tami, Chris Mole and David Wright.

In a joint statement, four of the group said Mr Blair had “not ended the uncertainty over when you intend to leave office, which is damaging the government and the party”.

Mr Watson, who has already been replaced as a junior defence minister by junior transport minister Derek Twigg, said he no longer believed that it was in the best interests of the party or the country for Mr Blair to continue.

But Mr Blair hit back saying that calls for his resignation were putting Labour’s electoral prospects at risk.

He said he had been “intending to dismiss” Mr Watson anyway “but wanted to extend to him the courtesy of speaking to him first”.

The Prime minister added: “Had he come to me privately and expressed his view about the leadership, that would have been one thing.”

“But to sign a round robin letter which was then leaked to the press was disloyal, discourteous and wrong.”

“It would therefore have been impossible for him to remain in government.”

<B>“Change wanted”</B>

The Sun newspaper said earlier that Mr Blair will go on 31 May, but Number 10 has insisted it had not leaked the date.

Meanwhile, leading Brownites are calling for the Prime minister to make a public declaration about when he will go, so that he cannot go back on it.

One of the chancellor’s closest allies, who asked not to be named, told the BBC News website: “From Gordon’s point of view there has to be a public statement Blair has to let it be known when he will go and there has to be a public declaration.”

<B>“Worst time”</B>

Ex-minister Doug Henderson, also a Brown supporter, said he could not see what Mr Blair would achieve by staying on.

“There should be a new leader in place by the end of March,” said Mr Henderson, in time for the local elections and mid-term polls in Scotland and Wales.

He said the timetable suggested on Tuesday by allies of Mr Blair – that he would go in about 12 months time – seemed “the worst time to appoint a new leader”.

“People in the country want a change,” he told the BBC News website. Ex-Cabinet minister David Blunkett said it was now “pretty clear that there is an understanding about a date next year - whether it’s 31 May when he announces it is open to speculation”.

Conservative leader David Cameron said the government was “in meltdown”, while Lib Dem leader Sir Menzies Campbell added that Mr Blair “should either resign or state a date”.