DAVID Miliband said today he will be standing as a candidate for the Labour leadership.
In a statement outside Parliament just before 5.30pm, the former Foreign Secretary said he will launch his campaign next week in his South Shields constituency.
He said the task for the party was to present itself again as "an alternative government".
Surrounded by a handful of supporters on the steps of the Houses of Parliament, Mr Miliband: "We now have a contest to succeed Gordon as leader of the Labour Party.
"I will stand as a candidate. I do so with humility in face of the responsibility this post brings and passion for the causes and values that led me to join our party."
Earlier, former defence secretary Bob Ainsworth suggested he may back Miliband in the battle.
He said: "I have worked very closely with David in our previous briefs and I have been impressed with him.''
But he pledged to evaluate every candidate and examine what ``contribution" they could make, adding: ``I may even nominate somebody who I am not going to vote for, to ensure that we have an open contest."
Mr Ainsworth also called for a swift process, saying: ``What we need to do is to get the balance right on the leadership contest between having an inclusive process and being able to involve as many people as we can, and the timetable.
``I don’t think we need an artificially prolonged timetable - we can get a new leader in place by July.
``We have to avoid what was unarguably an error two years ago and have any kind of coronation. We have to have a welcoming and open process, and it has to be substance not show."
No candidates have yet officially declared themselves contenders in the race to replace Mr Brown, though former foreign secretary David Miliband is expected to do so later today.
Former schools secretary Ed Balls is thought to taking his time before deciding whether to stand, while Jon Cruddas - a potential standard-bearer for the left - is actively considering a leadership bid.
Other possible candidates include Mr Miliband’s brother Ed, former health secretary Andy Burnham and party ``greybeard" Jack Straw.
Former home secretary Alan Johnson was among a raft of MPs who ruled themselves out of standing, along with former work and pensions secretary Yvette Cooper, former Wales secretary Peter Hain and former communities secretary John Denham.
Ms Harman urged her party to support their chosen candidates but be ``generous to the others".
``This is a contest within a team," she told them.
Former home secretary Alan Johnson earlier ruled himself out as a contender for the Labour leadership, announcing he will back David Miliband in the upcoming contest.
Mr Johnson had long been tipped as a possible successor to Mr Brown and was one of the leading Cabinet cheerleaders for a coalition with the Liberal Democrats which could have resulted in him taking the top job.
But this morning he said that Mr Miliband, the former foreign secretary, was the party’s ``greatest talent" and should be Labour’s new leader.
Asked if he was standing in the leadership contest, Mr Johnson told BBC Radio 4’s Today programme: ``No, I’m not. I am going to support David Miliband."
David Miliband is the first out of the traps in the race to replace Gordon Brown as Labour leader.
The leading Blairite has left little doubt of his ambition, though some insiders question whether he has the personal steel and the breadth of support in the party he would need to seize the crown.
Mr Miliband, 44, was a close ally of Tony Blair from the earliest days of New Labour, working for him in opposition from 1994 and heading the Number 10 policy unit during his first term in power.
Even before he became an MP, he was a key figure in reshaping the party’s agenda, nicknamed ``Brains" by Alastair Campbell for his powerful intellect and mastery of policy detail.
Parachuted into the safe seat of South Shields shortly before the 2001 election, he was a minister within a year and joined the Cabinet in 2005, rising to become Environment Secretary, where he put the issue of climate change firmly on the agenda for the first time.
When his former mentor quit 10 Downing Street in 2007, Mr Miliband’s appointment to one of the great offices of state reflected Mr Brown’s determination to mend fences with the Blairites and include talent from all sides of the party in his administration.
As the youngest Foreign Secretary since David Owen in 1977, he presided over a move away from "war on terror" rhetoric, but faced an awkward court battle over secret papers on Guantanamo Bay detainee Binyam Mohamed. He also ruffled feathers in India and Sri Lanka with attempted interventions in the Kashmiri and Tamil conflicts.
Doubts over his commitment to Mr Brown were sparked by a notorious press article days after a disastrous by-election in the summer of 2008, in which he discussed the future of Labour without once mentioning the Prime Minister.
The article was widely perceived as disloyal and triggered calls for his dismissal. It also meant that all eyes were on Mr Miliband at Labour’s annual conference in Manchester, where he appeared to revel in the attention, memorably posing for pictures with a banana.
But Mr Miliband did not mount a challenge, using his conference speech instead to hail Mr Brown’s leadership - the first of several occasions when he has been accused of ``bottling it" and tarnishing his hopes of eventually claiming the crown.
When his close friend James Purnell quit the Cabinet in June 2009, he took to the airwaves to reject his call for Mr Brown to stand down for the good of the party.
And in January this year, he held back from a resignation which could have given devastating momentum to an abortive putsch by former Cabinet colleagues Patricia Hewitt and Geoff Hoon.
Speculation that he had considered lending his weight to the Hewitt/Hoon plot was fuelled, however, when he waited a full seven hours before emerging with a decidedly lukewarm expression of support for the PM.
The son of Marxist intellectual Ralph Miliband and brother of fellow Cabinet minister Ed, the Foreign Secretary’s entire life has been bound up in left-wing politics.
Before he formally announced his decision to stand he had already won the backing of former home secretary Alan Johnson, who was seen as a possible contender for the leadership.