Tony Blair's strategy of refusing to name his departure date was in danger of backfiring on Friday, with his stance provoking previously loyal Labour MPs into open dissent.

The reaction of two new Labour backbenchers typified the widespread expressions of concern provoked in disparate wings of the party.

Lyn Brown and Sarah McCarthy-Fry, both elected only last year, called for clarity on the succession timetable: exactly the demand that Mr Blair had hoped to quash with this week's insistence in an interview with The Times that he would not set a date for his departure.

The two MPs warned that Mr Blair's latest attempt to stop the tide of debate about the succession, which has ebbed and flowed since his 2004 promise not to fight a fourth general election, was doomed to failure.

"You can't put the genie back in the bottle. There is no way to stop the speculation from happening," Ms Brown said.

Ms McCarthy-Fry lam-ented that Labour was now "stuck in this quagmire and we cannot get out of it because everything swings back to the leadership",

The backlash against Mr Blair's announcement spread well beyond the new parliamentary intake. Nick Raynsford, a former minister, predicted that a "large number" of his fellow Labour MPs would be "surprised and disappointed by this".

Andrew Smith, the Brownite former work and pensions secretary, argued the prime minister's stance would provoke "widespread concern" within the party. "I would have thought that it's clear to everyone that the debilitating uncertainty over the leadership can't go on," Mr Smith told the BBC.

Blairite loyalists fought back, claiming it was unreasonable to expect the prime minister to make a statement that would instantly render him a lame duck. David Blunkett, former home secretary, said: "The moment a timetable is set, you might as well have announced you are going now."

But such staunch defenders appeared in a minority. The breadth and intensity of the backlash suggests Mr Blair faces, at best, an uncomfortable few weeks over the period of the Trades Union Congress and Labour party conferences.

Tony Woodley, general secretary of the Transport and General Workers' Union, warned the status quo was not sustainable.

"We've got confusion, we've got disillusionment and we've got dissatisfaction within the Labour party," Mr Woodley told GMTV.

Calling for a change in the leadership "sooner rather than later," the union boss warned the uncertainty would help the Tory resurgence in the opinion polls and "that's really dangerous for Labour."

George Mudie, Labour's former deputy chief whip, said the party conference in Manchester at the end of the month was "the natural place to ask the leader what's going on".

The big fear for Downing Street is that the party conference will prove the catalyst for an orchestrated rebellion, rather than simply prime ministerial embarrassment.

One potential source of such a rebellion, the 32-strong group of Welsh MPs who will meet on September 18 to discuss tactics, were vociferous in their reaction to Mr Blair's announcement.

"I can't think of a single Welsh MP who does not want him to go now," Paul Flynn told the FT. He cited a meeting some of the MPs had with the prime minister in June to discuss their fears that Mr Blair would damage Labour at next May's elections for the Welsh assembly.

"He said 'what's the problem guys?' and the first three of us who spoke said 'you're the problem'."

Even among MPs willing to tolerate the lack of a timetable, most were clear Mr Blair's days in Downing Street are numbered.

Mike Gapes, chair of the Commons foreign affairs committee, said he was "reasonably content" with Mr Blair's statement because of its lack of ambiguity. He said: "I don't think there's any other interpretation than that he will go next year,"