Mr Blair has dramatically failed to quell the rebellion among his main union critics and can expect to experience a rough ride at the TUC Congress next week.
In his short statement yesterday, he acknowledged that next week's TUC Congress would be the last he attends as party leader. That, he said, would probably come as "a relief" to both the unions and himself.
But Derek Simpson, leader of Amicus, Labour's biggest single financial donor, said the Prime Minister's promise to go within 12 months failed to deal with the leadership crisis.
"The statement is not good enough if we have to endure another year of failed policies on jobs, pensions, the National Health Service and education. He has turned off Labour support. Membership has halved, activists who knock on doors have dwindled to almost nothing and that will not change as long as Tony Blair is prime minister."
An Amicus survey showed that three-quarters of its workplace representatives wanted an immediate change of leadership.
Mr Simpson said: "The current mess in Westminster is the tip of the iceberg.
"Our workplace reps are Labour's foot soldiers and they are now in open revolt, such is their frustration over government policy on the issues that affect them, their colleagues and their communities. Unless something changes, Labour will lose the next election."
Mr Blair faces one of the toughest public appearances of his premiership when he takes a question and answer session before 800 hard-bitten trade unionists at the annual TUC Congress in Brighton.
Delegates at the assembly - many of whom are grassroots Labour activists - are expected to insist that the Prime Minister publicly sets out the timetable for his departure rather than rely on a vague promise to go within the year.
The TUC general secretary, Brendan Barber, yesterday set the tone for Mr Blair's appearance in the lion's den next Tuesday by in effect urging the Prime Minister to make up his mind and name the day.
"As the Prime Minister continues to consider this - and it is for him to decide - I'm sure he'll be reflecting on the dangers of prolonged uncertainty undermining the Government's effectiveness."
Senior sources at the Transport and General Workers' Union registered scepticism about Mr Blair's statement. "If the politicians have reached an agreement, unions shouldn't rock the boat, but there must be scepticism that this settlement is durable for more than the short-term."
The most supportive views Mr Blair could muster among unions affiliated to Labour were from the ultra-loyalist shopworkers' union Usdaw, and the steelworkers' union Community, which both said that they had "no view" on the issue.
The Trades Union Congress has no constitutional links with Labour. But asked about indications that the Prime Minister might depart next May, Mr Barber said that under those circumstances, "it would be difficult to take forward in a positive way some of the key areas the Government needs to address".
The Prime Minister has indicated that he had no intention to use the conference season to announce his timetable for departure.
Mr Barber said that while the union movement had differences of opinion with the Chancellor, the TUC leader backed Mr Brown for the Labour leadership.
How long can he hang on?
Tony Blair's best-case scenario is that yesterday's promise to go within a year is enough to satisfy his Labour critics and secure a working relationship with Gordon Brown.
He wins backing from union leaders next week at their annual congress, warm recognition from Labour activists of his record in office and a ceasefire within his parliamentary party. He quits as Labour leader on 31 May, handing the keys to Downing Street to the Chancellor on 26 July.
The Prime Minister has done enough to quell the immediate revolt and, despite some noises off, gets through the conference season with credit.
But there is little enthusiasm among his troops and no sign of recovery in the opinion polls. A bleak night for Labour in the Scottish and Welsh elections confirms that he is box-office poison for voters and he announces his resignation on the following day, 14 May. That allows his replacement to be crowned in late June.
The muttering fades but never vanishes. Mr Blair gets a lacklustre reception at the TUC and Labour conferences and the mood of rebellion is palpable when the Commons returns on 9 October.
Mr Blair realises the game is up at Christmas and limps on until the Labour spring conference in Glasgow, announcing his resignation on 19 February. The new leader is in place by early April, enabling Labour to present a new face to voters in the May election.
Mr Blair buys time with a barnstorming speech to the Labour conference. But Labour MPs return to Westminster in mutinous mood and immediately start plotting Mr Blair's downfall.
The haemorrhage of support forces him to abandon his dream of 10 years in power and he announces his resignation on 18 November. A new leader takes over in the new year.
Mr Blair is heckled at the TUC Congress and faces hostile motions at the Labour conference. More ministers resign and Gordon Brown openly unleashes his attack dogs. On 26 September Mr Blair announces: "I am resigning."
By Nigel Morris and Ben Russell
Patricia Hewitt Health Secretary
"We can now... get back to what really matters - improving the lives of the people of our country."
Chris Bryant Organised letter calling for Blair's resignation
"I hope we can get on with... appointing our new leader sooner rather than later."
Derek Simpson General Secretary of Amicus
"Our workplace reps are Labour's foot soldiers and they are now in open revolt."
Peter Mandelson EU Trade Commissioner
"I hope that the plotting and the shenanigans will be put behind them once and for all."Reuse content