In a statement last night, Mr Livingstone stressed that he would "stand on the manifesto agreed by the Labour party, as must every candidate".
Frank Dobson, Tony Blair's favoured choice, raised the stakes when he hinted that he could pull out of the contest if Mr Livingstone is blocked today.
The former health secretary's threat came as a new controversy erupted over the leaking by Millbank of transcripts from Mr Livingstone's interview with the party's London Selection Board on Tuesday.
According to the leak, the Brent East MP told the panel that he would withdraw from the contest if a manifesto was drawn up which he disagreed with. "If I couldn't accept the manifesto, I would stand down as a candidate," he is reported to have said.
Mr Livingstone appeared to complete a volte-face on the issue last night in his statement.
"If there is any doubt on this, let me spell it out. If selected as Labour's candidate for London mayor, I will stand on the manifesto agreed by the Labour Party. I would not dream of withdrawing if I were to be chosen as Labour's candidate," he said.
However, the MP repeated his opposition to the Government's general election manifesto pledge to stage a public-private partnership for the London Underground. He said that although he would work with the party to draw up the mayor's manifesto, he would continue to campaign against what he calls the partial privatisation of the Tube.
His last-minute decision to tone down his stance followed a day of hardline statements from Millbank and from the Cabinet. In a move that clearly rattled the MP, senior sources stressed he would be barred if he repeated his line when recalled before the panel today.
John Prescott, the Deputy Prime Minister, welcomed Mr Livingstone's apparent "conversion" last night, but said no candidate could be allowed to have a veto over the manifesto. "If you run as a Labour candidate you run on Labour manifestoes which are decided by the party," he told Channel Four News.
"Ken wants to be a Labour candidate, the Labour Party requires you to sign up to a manifesto. We can't have candidates who come along and pick and choose which parts of the manifesto they want." Mr Prescott added that the proposal for a public-private partnership for the Tube had already been discussed and agreed by the party. "This is a party policy, have no doubt about it," he said.
The danger of Labour's selection contest collapsing into farce became real last night after Mr Dobson told friends that he would pull out of the whole race if Mr Livingstone was blocked. A spokesman for Mr Dobson said that he would "consider his position" if the panel did bar his rival.
The Dobson camp claimed that their canvassing showed they had pulled ahead of Mr Livingstone.
Out of 11,298 party members canvassed 4,188 said they supported Mr Dobson and 3,770 said they backed Mr Livingstone. Only 277 said they backed Glenda Jackson while the rest were "don't know", "won't say", or "would not vote", according to Mr Dobson's campaign headquarters.
Campaign manager Nick Raynsford said: "We want Ken Livingstone to go forward into the ballot.
"We know we can beat him and if Ken Livingstone is now angling to run as an independent it can only be because his own telephone canvassing operation - begun last week - is showing him the same results."
Mr Livingstone said earlier that he was "amazed and appalled "at the decision to leak his comments to the panel.
"I make a simple challenge now to the people who are leaking those titbits: let everybody see the full transcript, the whole thing from start to finish, not selectively, half a sentence here, half a question there.
"I think the public will be genuinely shocked at some of the aggressive questioning that I was subjected to," he said.Reuse content