Environment Secretary David Miliband has announced that he will vote for Gordon Brown as Labour leader.
Mr Miliband says he won't change his mind about the leadership
His declaration will be seen as giving a boost to the chancellor, the favourite to succeed Tony Blair.
When Mr Miliband told the BBC on Tuesday that he did not intend to run for the top job, critics said he had left himself room to change his mind.
But he tries to remove any uncertainty when he writes in The Observer: "I will vote for Gordon Brown".
After months of intense speculation, Mr Miliband told the BBC he did not intend to run for the Labour leadership when Mr Blair announces his resignation after the 3 May elections.
In The Observer, Mr Miliband seeks to make it absolutely clear that Mr Brown is the best candidate to take forward Mr Blair's legacy.
He argues that success in the next general election will require the party not to go back on the New Labour project, but to offer "New Labour Plus".
"I will vote for Gordon Brown to lead Labour's drive," he writes.
"I have watched him and worked with him for nearly 20 years. He has in the last 10 years done great things for living standards; no-one is better qualified to lead across a wider canvas."
He adds: "I said three years ago that I would not be a candidate for the leadership. I meant it and have not wavered from that view.
"I certainly am not in the business of waiting to pounce on local or Scottish and Welsh election results to change my mind."
Attorney General Lord Goldsmith told Sky News: "We are going to have a very, very strong prime minister, I am absolutely confident about that, behind which the Labour Party I have no doubt will unite. That is what is important.
"I am talking about Gordon Brown, absolutely. I am sure that he will be the next prime minister."
Candidates need the support of 44 MPs to join the leadership contest.
So far only backbenchers Michael Meacher and John McDonnell have said they will stand.
Mr Meacher, the former environment minister, said he had the support of 25 Labour MPs and expected to inherit 15 more who were currently backing Mr McDonnell, the chairman of the Socialist Campaign Group.
Mr Meacher said: "We should not have a coronation when there has been no debate on policy since Blair came in in 1994.
"It's crucial that Brown is made to answer as to what he's going to deliver to us. We are entitled to know."
New Labour Plus
Some commentators believe that there could also be challenges from Home Secretary John Reid and former home secretary Charles Clarke.
Mr Reid appeared to keep his options open in an interview with GMTV's Sunday Programme.
He said: "By saying that I won't discuss it, by definition you don't rule in or out.
"I'm sure the prime minister will make his views more specific, and when he does that, we will all have the chance to make sure that we express our views."
Mr Miliband said New Labour Plus would be "broader and deeper" than New Labour.
It would bring together what he regards as the good things about New Labour and new policies such as creating institutions to bring citizens together, action to ease access to the housing market and the devolution of money-spending power to a local level, he added.