Tony Blair has tried to reassure MPs by saying he will give his successor time to "bed in" before the next election.
Mr Blair says he wants to get on with the business of government
He rejected calls for him to set a date for his departure, saying this would "paralyse the working of government" - but promised a "stable transition".
Some backbenchers welcomed Mr Blair's words but MP Graham Stringer said he was still causing instability.
A new opinion poll suggests support for Labour is now at 30%, equal to its lowest level since 1992.
The Populus poll in The Times puts the Tories on 38% and the Liberal Democrats on 20%.
It also suggests Conservative leader David Cameron, who says the sooner Mr Blair goes the better, is preferred to both the prime minister and his likely successor, Gordon Brown.
Monday evening's meeting of the Parliamentary Labour Party followed Labour's heavy losses at last week's local elections.
Mr Blair told MPs he wanted his "legacy" to be a fourth term Labour government after his time as prime minister.
He has previously said he planned to serve a "full term" of office.
But on Monday he said he would leave in time for his successor to settle he would give "the time properly needed for my successor to establish themselves".
Some MPs pressed Mr Blair on what time gap he meant.
And a Labour spokesman said some people had expressed "elements of dissatisfaction with the election results" but the "vast majority" of MPs and peers backed Mr Blair's stance.
He said Mr Blair's message to them had been "I'm determined that what I've set out to you will happen... let's work together to make sure it does happen".
Mr Stringer said the prime minister had accused some of those calling for a departure timetable of always being opposed to him.
The MP said that created a "certain level of anger and resentment" and it was currently difficult to see how unity would be achieved.
Fellow MP John Grogan said he had been reassured by Mr Blair, who had moved "quite a lot" by saying he would allow "ample" time for his successor.
Backbencher Geraldine Smith said Mr Blair was "moving in the right direction".
But she said the prime minister had not answered her question when she asked him what he meant by giving "ample" time for his successor to settle in.
Earlier, at his monthly news conference, Mr Blair refused to say whether he had discussed the timing of his departure with Mr Brown.
He said some of those calling for him to go wanted to "radically change the direction of policy and not to renew Labour but to reverse it".
That would consign Labour to opposition and deny it the chance of a fourth term in government, he told reporters.
Mr Blair said that unlike former Conservative Prime Minister Margaret Thatcher he had "no intention of going on and on and on".
But he said: "To set a timetable now would simply paralyse the proper working of government, put at risk the necessary changes we are making for Britain and damage the country.
"It wouldn't end this distraction, but it would take it to a new level."
The prime minister said it was in Labour's interests to "get on with the business of government".
But Liberal Democrat Sir Menzies Campbell urged Mr Blair to put the country's interests above his own and set a departure deadline.