Alan Johnson confirmed that he would not challenge Gordon Brown to succeed Tony Blair as he joined the increasingly crowded race for the party's deputy leadership.
The Education Secretary, who was once seen as a potential leadership rival to Mr Brown, heaped praise on the Chancellor as he finally announced his bid for the deputy's chair.
Allies said Mr Johnson had amassed 70 supporters among Labour MPs and MEPs, well over the 44 needed to secure a place on the ballot paper.
Mr Johnson, a former postman who rose to lead the Communication Workers Union, will formally launch his deputy leadership campaign in his Hull constituency today pledging to launch a two-year drive to retain Labour's vulnerable marginal seats.
He joins a crowded field, with Hilary Benn, the International Development Secretary, Peter Hain, the Northern Ireland Secretary, and Harriet Harman, the Constitutional Affairs Minister all declared candidates, along with backbencher Jon Cruddas, a former Downing Street adviser turned East End MP.
Jack Straw, the leader of the Commons, has also not ruled out a bid for the deputy leadership, although he has not declared his intention to stand.
Yesterday Mr Johnson, 56, said Mr Brown was "a towering political figure". He told the BBC: "I think actually there is a feeling throughout the party that Gordon is a towering political figure.
"It is, of course, a matter for the party to decide who the leader going to be, but my view is that Gordon is a towering political figure. It is not a position I ever sought and there is a superb candidate in Gordon Brown."
He insisted that he did not want to challenge for the leadership. He said: "You have to first of all decide whether you want to do it. Secondly, you have to decide whether - if you want to do it - is there someone much better for the job?
"On the first question, I didn't want to do it, and on the second question, there is someone vastly superior."
Mr Johnson's announcement saw bookmakers install him as a virtual certainty to win, with odds on a Brown succession being slashed to 1/10.
Mr Johnson will today pledge to "assist, support, cajole and complement the person who carries the heaviest of all burdens".
He released messages of support from a series of MPs including Sadiq Khan, the Tooting MP, Gerry Sutcliffe, a Home Office Minister, Laura Moffatt, the Crawley MP and John Grogan, MP for Selby.
In an open letter to Labour members, Mr Johnson said he was, "someone whose background, foreground, mainland and hinterland are working class".
He wrote: "I am a product of the trade union movement. Not an unquestioning sentimentalist - agreeing with a proposition because it happens to be made by a trade union leader. But a believer in the fundamental good sense and decency of working people, who would have little voice or influence in our society without the Unions' vital link with the Party."
Meanwhile, Michael Meacher, the former environment minister, signalled that he would challenge Mr Brown for the leadership with a campaign based on radical action to combat climate change.
Mr Meacher launched a new Labour campaign group calling for "Big Change" in the direction of the government over global warming. He threatens to take votes away from the other challenger from the left, John McDonnell.
Mr Meacher, the environment minister for eight years, had a handful of leftwing MPs including Alan Simpson and Ian Gibson at the launch of the campaign.
Although he said the time for announcing a challenge would come later, he said he was not denying he was running for the leadership. "I do believe there should be a candidate on the centre left which I think is the majority of the party," he said.
"If Labour is going to win the next election, we need a fundamentally different direction of travel for the new Government after the election of a new leader and deputy," he said. "We need more than corrections of where we have clearly gone badly wrong over Iraq, Lebanon and subservience to [United States president George] Bush."Reuse content