Here is a rundown of the five candidates in the race to be the next Labour leader.
David Miliband was the first of the likely contenders to replace Gordon Brown to throw his hat into the ring. He says he wants Labour to "rebuild itself as a great reforming champion of social and economic change" in what he acknowledged was a "new era" in British politics.
And signalling his intention to fight on the Blairite centre ground he said that with the Lib Dems in coalition with the Tories he believed Labour can be a "great unifying force on all shades of centre left opinion in this country".
He has been at pains to avoid publicly clashing with younger brother Ed, insisting their "brotherly love" will survive the contest - even as it became increasingly apparent that Ed was his main rival for the Labour crown.
Tensions have surfaced however, as Ed seeks to appeal to Labour's core support, and David warns against the party retreating into its left wing "comfort zone," stressing the need to appeal to a broad cross-section of voters.
David Miliband was appointed foreign secretary in June 2007, becoming - at 41 - the youngest person to hold the post for 30 years, after establishing a reputation for competence as environment secretary.
Often spoken of as a possible Blairite challenger to Mr Brown, he decided against standing for the Labour leadership when Tony Blair quit.
After doubts began to grow within the Parliamentary Labour Party about Mr Brown as PM, Mr Miliband's supporters again urged him to mount a coup in 2008 - although he eventually made clear that he would not.
These decisions did not create a reputation for decisiveness or fearlessness.
Mr Miliband's intellectual ability is widely admired but his presentational skills were questioned when he allowed himself to be photographed - at the height of the speculation over a potential coup - grinning and holding a banana.
Roughly the same age as David Cameron and Nick Clegg, Mr Miliband became an MP in 2001 after working as head of the Downing Street policy unit.
Ed Miliband cut his political teeth as a member of Gordon Brown's inner circle of special advisers.
He worked for Mr Brown during his early years as chancellor, helping with long-term policy development on economic and social policy issues, including taxation, public spending and the labour market.
In June 2007, at the age of 37, and only two years after becoming an MP, he was drafted into the cabinet with responsibility for the beefed-up Cabinet Office, which co-ordinates policy between departments. He also had the job of writing Labour's election manifesto.
As energy and climate change secretary his profile rose, but was still lower than that of his brother.
Mr Miliband joined the Labour Party aged 17 and went on to get an economics degree at Oxford University, before studying as a postgraduate at the London School of Economics.
He says he thought long and hard about standing against his older brother, who has insisted "brotherly love will survive".
Positioning himself firmly to the left of David, he has warned the party against retreating into a "New Labour comfort zone" - widely interpreted as a swipe at his older brother.
It is a strategy that has seen him secure heavyweight backing from Britain's biggest trade union leaders. The GMB has even threatened to withdraw funding from the Labour Party if he does not win the contest.
Ed Balls has long been Gordon Brown's closest ally, with all the pluses and minuses that involves.
One positive was quick promotion, being appointed schools and children secretary in June 2007, just two years after becoming an MP.
This followed 10 years as Mr Brown's chief economics adviser at the Treasury, playing a critical role in a string of big policy decisions.
Mr Balls has a tough, combative approach to politics which some of his enemies regard as confrontational and bruising.
Given his closeness to the former PM, he has many of the same enemies and could be portrayed as the "no-change" candidate. Announcing his campaign he said the contest was not about "Blair versus Brown" or "old Labour versus new Labour".
"I think that's the past really - what people want to know is, are we in touch with the public, are we on their side, do we understand their concerns?," he said.
Mr Balls is married to fellow ex-cabinet minister Yvette Cooper.
Andy Burnham is a relatively new figure to the general public, having been an MP for nine years and a cabinet minister only since the start of Gordon Brown's three-year tenure as prime minister.
But he has been on the Labour scene for many years, working as a researcher to arch-Blairite Tessa Jowell in the mid-1990s.
One of the more affable figures in the last Labour cabinet, the Liverpool-born and Cambridge University-educated father-of-three is a football fan and keen musician.
As Health Secretary Mr Burnham handled one of the biggest-spending departments in Whitehall, but his youthful appearance and lack of wide experience may raise questions over his readiness to make the leap to party leadership.
However, before taking on the health brief Mr Burnham, the MP for Leigh, was culture secretary and chief secretary to the Treasury.
Although widely seen as a supporter of Tony Blair, he remained loyal to Mr Brown during his premiership, declaring himself a "Blairite for Brown".
Diane Abbott announced she was entering the race because she said there was little to choose between the other contenders.
Britain's first black woman MP, Ms Abbott has been in the Commons since 1987 and increased her majority at the general election.
She, along with Tory Michael Portillo, is a resident pundit on BBC One's This Week Show and frequently voted against the party leadership during the Blair-Brown years.
Ms Abbott, the only left-wing candidate following the withdrawal of John McDonnell, has said her bid is "serious". She has set out a policy platform distinct from the other contenders, including higher taxes for the rich and scrapping Trident nuclear weapons.
She has also tried to turn her lack of experience at the highest level - she has never held a frontbench role - to her advantage, by portraying herself as a fresh start for the party after in-fighting of the New Labour years. She is the only candidate to have consistently spoken out against the Iraq war, a point she has been quick to stress at leadership hustings.
On her website, the Hackney North and Stoke Newington MP said: "However long this run lasts, I am glad that I have been able to make my point."