The new Home Secretary Alan Johnson today insisted he was not seeking the leadership of the Labour Party, and said he was backing Gordon Brown "to the hilt" to continue as Prime Minister.
Mr Johnson said he would "never say never" to becoming Prime Minister at some point, but insisted he could see no circumstances at present where he would mount a bid for the post.
He insisted that Mr Brown was "absolutely the best person for this job".
Mr Johnson is the bookmakers' favourite to replace Mr Brown if he steps aside, but has been careful to avoid any suggestion that he is angling for a move to 10 Downing Street.
Following his move from Health Secretary to the Home Office in today's reshuffle, Mr Johnson told Sky News: "I am supporting Gordon Brown. I am backing Gordon Brown. I have no ambition to be the leader. I have never had any ambitions to be the leader."
Many Labour activists and some MPs are thought to believe that the party would do considerably better in the coming election with Mr Johnson at its helm, and he is understood to be the rival most feared by David Cameron's Conservatives.
But Mr Johnson said: "I'm long enough in the tooth to realise that these things have to be taken with a pinch of salt, not to get carried away by these things.
"I am flattered that people think highly of me. I want them to think highly of me as a Home Secretary in a Labour Government serving under the Prime Minister.
"I do genuinely believe - and I think I am in the majority here within the party and within the Cabinet - that Gordon is the best person for this job, absolutely the best person for this job."
Asked if he was ruling out ever seeking the Labour leadership and the post of Prime Minister, Mr Johnson replied: "I'm not saying under no circumstances would I ever run, because if you talked to most politicians they would never say never.
"But in terms of the position we are in at this moment, at this time, I can see no way that I would ever be Prime Minister and I'm certainly backing this Prime Minister to the hilt."
Mr Johnson rejected the claim of former Work and Pensions Secretary James Purnell that Labour would have a "fighting chance" of winning the election only if Mr Brown stood aside.
"I disagree first of all with his analysis that we are less likely to win the next election under Gordon Brown," he said. "I think we are less likely to win the next election if we show complete disunity and we need to get a grip of this.
"We need to recognise that Gordon Brown is leading this country through some of the most turbulent times, a real economic maelstrom around the world that he is dealing with.
"On the other hand, the issue of Members' allowances has damaged all parties, dealing with that as well and tackling the issues like crime and health and education that are the bread and butter issues. It is a difficult job at the best of times and it is not a job that his own colleagues should be making more difficult through their actions."