Tony Blair has been warned by a top union leader not to repeat the mistake of Margaret Thatcher by staying on too long as prime minister.
Tony Woodley, head of the Transport and General Workers Union, urged an end to the uncertainty about Mr Blair's plans.
His appeal came as Labour MPs voiced anger and frustration at Mr Blair's refusal to use this month's Labour conference to give an exit timetable.
Mr Blair has told his critics: "Let me get on with the job."
And in a briefing with journalists in Scotland, he shrugged off comparisons with Mrs Thatcher's declaration to go "on and on", saying: "You pays your money and you takes your choice."
He said the last general election had only taken place 15 months ago and he wanted speculation about his future to end.
Mr Blair has already said he will step down before the next election but there is mounting pressure for him to go further and end speculation about his departure date.
Mr Woodley, leader of one of the big four unions, said those advising Mr Blair were not living in the real world.
"They should be advising him not to get himself into a position as Thatcher did, when she did not understand when to go," he told BBC News.
Labour was now the only party which did not have a leader to fight the next election, which was going to be very difficult to win, he argued.
Mr Blair should end the "confusion" sooner rather than later, added Mr Woodley.
His warning was echoed by Labour backbencher Geraldine Smith, who said Mr Blair should remember that nobody was bigger than the party.
People wanted to see Mr Blair leave on a high, not be dragged down like Margaret Thatcher, Ms Smith told BBC News.
"If he carries on, the prospects look gloomy for him because it's doing the Labour Party and the government so much damage," she said.
Earlier, ex-Cabinet minister Andrew Smith said uncertainty about the leadership was making the government "drift".
"I would have thought it's clear to everyone that the debilitating uncertainty over the leadership can't go on - it's bad for the country, bad for the government, bad for the Labour Party, and ultimately bad for Tony Blair himself," he said.
And former deputy chief whip George Mudie said Labour's conference later this month would be "very messy" if the uncertainty went on.
David Blunkett says Mr Blair must not set a timetable
Amid the disquiet, there is speculation that up to 80 backbenchers could send an open letter to Mr Blair demanding a departure timetable.
But former Cabinet enforcer Jack Cunningham said those raising the issue often had their own agendas and were persistently distracting attention from important policy issues.
And ex-Home Secretary David Blunkett said setting a timetable would be tantamount to stepping down now.
"The minute you say it, the whole of the civil service grinds to a standstill, all your power goes and government atrophies in a way it does not at the moment, despite the discussions that are going on," he said.
The Guardian newspaper said Mr Blair's "current thinking" was that he would stand down next summer.
But Mr Blair told the Times: "I have done what no other prime minister has done before me. I've said I'm not going to go on and on and on and said I'll leave ample time for my successor," he said.
"Now at some point I think people have to accept that as a reasonable proposition and let me get on with the job," he said.
"If what they are really worried about is timing I think most of you can look at what I have said and draw conclusions about that."