Miliband wastes no time in announcing his desire to succeed Brown

Ever since David Miliband stepped back from challenging Gordon Brown for the Labour leadership his critics have, rather unkindly, labelled him a ditherer.

But not yesterday. Moments after David Cameron and Nick Clegg announced their plans for government, Mr Miliband outlined how he planned to challenge them – becoming the first Labour figure to announce his candidacy for leadership of his party.

The former Foreign Secretary, who will hold a formal campaign launch in his South Shields constituency next week, is expected to be the front-runner when the contest gets properly under way. The contest could be unique in having brother pitted against brother, because Mr Miliband's younger sibling, Ed, is also calculating whether he has the support to make a credible candidate.

If some Labour MPs had their way, the contest would also feature a husband and wife running against each other; Yvette Cooper, the former Work and Pensions Secretary and the most talented woman politician in Labour's top team, is married to the ambitious former Schools Secretary Ed Balls, who is thought to be the person Gordon Brown would most like to see as his successor.

Yesterday, Ms Cooper effectively ruled that out when she told the BBC: "I'm not putting myself forward for the top job right now."

But some Labour MPs insisted after her statement that they wished Ms Cooper would change her mind in order to have a credible woman contender in the race, even if it meant persuading her husband to stand aside.

Mr Miliband, who appeared on the steps of Parliament to announce that he would be running, has the backing of almost the entire Blairite old guard, although the best-known figures, such as Tony Blair himself, Peter Mandelson and Alastair Campbell, are not likely to become involved in the contest.

Mr Miliband, 44, was a close ally of Mr Blair from the earliest days of New Labour, working for him in opposition from 1994 and heading the No 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 Mr Campbell for his powerful intellect and mastery of policy detail.

One of the MPs standing at Mr Miliband's shoulder yesterday was David Hanson, who worked closely with Mr Blair when he was Prime Minister. Mr Miliband also has the backing of the former Home Secretary, Alan Johnson.

However, he has also drawn support from newly elected MPs, four of whom joined him for yesterday's announcement including Britain's youngest MP, Pamela Nash, 25, who won the Scottish seat of Airdrie and Shotts.

But Ed Miliband, the former energy secretary, was conspicuously absent. He is "taking soundings" before deciding whether to run against his brother. The backbench MP Jon Cruddas, who came a creditable third in the 2007 deputy leadership race, is likely to run. Andy Burnham, the former health secretary, is also testing to see if he can muster the necessary support.

"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," David Miliband said.

He added a call for a "genuine, real contest with many contestants". Many senior members of the Labour party believe it was a mistake to allow Gordon Brown to assume the leadership in 2007 without one. The timetable of the leadership battle will be decided by Labour's national executive when it meets early next week.

It looks likely that they will decide to let it run until the party's annual conference, which opens on 26 September.

The contest could be foreshortened so that the new leader is able to take office in July, although that would put a strain on the Labour party's depleted bank balance because of the cost of holding a postal ballot and a special one-day conference to announce the result.

If it is delayed, it can be included with the postal ballot that has to be held anyway to elect a new national executive in September.

Labour MPs met for the first time since the election yesterday afternoon to be given a pep talk by Harriet Harman, who has taken over as interim leader. Ms Harman, who is not going to contest the leadership, pleaded for a clean contest. "We all know that the party expects, and is entitled to expect, the contest to be a credit to the party. Each contestant's supporters will be generous to the others. This is a contest within the team," she said.

Participants said afterwards that the MPs' meeting was surprisingly upbeat considering Labour lost 91 seats in the general election, mainly because they had feared the result could be much worse. "People talked as if we just won a great victory," one bemused participant said afterwards.

The odds

2/7 David Miliband

5/1 Ed Miliband

9/1 Ed Balls

8/1 Andy Burhnam

20/1 John Cruddas

50/1 Yvette Cooper

100/1 Peter Mandelson

Source William Hill