David Miliband is the bookmakers' clear favourite to replace Gordon Brown [EPA]
Gordon Brown, Britain's prime minister, has said he will step down this year to give his Labour party a chance of forming a government with the Liberal Democrats.
Brown said he intended to depart in time for a new party leader to be chosen before Labour's conference in September.
Here are some details on likely contenders for the position.
Born in July 1965, Miliband was nicknamed "Brains" by an aide to Tony Blair, the former prime minister, and has been foreign minister since 2006, the youngest in that post since David Owen in 1977.
After Blair stood down as prime minister in 2007, Miliband was touted as a possible challenger to Brown for the top job.
He was also briefly linked to a leadership challenge in 2008 before dismissing the media reports.
Miliband is the bookmakers' clear favourite to replace Brown.
Johnson moved from the health ministry to the more powerful interior ministry in June 2009.
Born in May 1950, he is portrayed as a man of the people who could help Labour reconnect with its working class roots.
His easy style impresses supporters, but critics say he is vague on policy, particularly on the economy.
Johnson was orphaned at the age of 12 and brought up by his elder sister in a state-owned flat in London.
A guitar-playing former postman, he worked as a supermarket shelf-stacker before becoming a trade union leader.
Johnson has ruled himself out of standing for the leadership and has said he will support David Miliband.
Brown's former right-hand man at the finance ministry, Balls was appointed secretary of state for children, schools and families in June 2007.
Born in February 1967, he was educated at Oxford and Harvard universities and worked as a leader writer and columnist at the Financial Times.
Balls is married to Yvette Cooper, the work and pensions secretary, and is considered to be firmly on the left of the party.
Younger brother of David, Ed was born in December 1969 and is seen as one of the rising stars in the party.
Supporters say he has a more relaxed presentational style than his brother and that would make him the best candidate to deal with David Cameron, the Conservative leader, who is often portrayed as a polished performer.
He was named energy and climate change secretary in 2008.
|Harriert Harman - ruled out
Born in July 1950, Harman was educated at St Pauls, one of Britain's elite girls' schools.
She worked as a civil liberties lawyer before entering parliament in 1982.
The leader of the lower house and deputy Labour leader, she has close ties to Brown.
Supporters describe her as a tenacious workaholic with a strong record on sexual equality and workers' rights.
Harman was the first contender to rule herself out following Brown's announcement, saying she wished to remain as the party's deputy leader.
No ministerial experience, and on the left of the party, but highly regarded by moderates too.
Dubbed the "Thinking man's street fighter" in an article by the Economist magazine, Cruddas was born in April 1962.
He represents a constituency in east London where he has gained kudos for campaigning against the far-right British National Party which has a large support base in the area.
Jack Straw, the justice secretary, was born in Essex in August 1946 and read law at the University of Leeds.
A member of Blair's first cabinet, he is one of only three people to have served in cabinet continuously since Labour came to power in 1997 and has held the positions of both interior minister and foreign minister.
In 2009, a poll showed Straw was the public's choice to replace Brown and he was quoted as saying: "If someone said, 'Sign on the dotted line', yes, I'd sign."
In April, Straw, indicated he was personally against offering radical voting reforms to the Liberal Democrats, saying the proportional representation the smaller opposition party sought had undesirable consequences.
Straw has ruled himself out of standing for the leadership.
Born in November 1953, the Edinburgh Central MP attended the private Loretto School on the outskirts of Edinburgh, before going on to study law at Aberdeen University.
Darling is a former left-winger who moved to the centre ground when Blair revamped Labour during its long years in opposition.
He took over as finance minister when Brown became prime minister in 2007.
|Peter Mandelson - ruled out
Mandelson, born in October 1953, was one of the architects of Blair's "New Labour" project, which moved the Labour Party to the political centre, enabling it to win a landslide election victory in 1997.
Currently secretary of state for business, innovation and skills, he served as MP for Hartlepool for 12 years from 1992, before vacating his seat to become a European commissioner between 2004 and 2008.
Mandelson sits in the unelected House of Lords, which would prove an obstacle to him becoming prime minister.
Famous for his tactical skills as a backroom operator, Mandelson has long been nicknamed the "Prince of Darkness".
Mandelson has ruled himself out of standing for the leadership.
The rules governing the election of a Labour leader are complicated and depend on whether there is a vacancy or not.
When the party is in government and a vacancy arises, the cabinet will, after consultation with the party's National Executive Committee, appoints one of its members as interim leader until a ballot can be held.
If the party is in opposition, the deputy leader automatically becomes temporary leader.
To take part in the leadership election, any potential candidate must be supported by 12.5 per cent of the members of the Parliamentary Labour Party.
The voting is split equally three ways between Labour MPs and the Labour Members of the European Parliament; party members; and members of affiliated trade unions who have not opted out of paying a political levy.
The votes of each nominee in each section are then calculated as a percentage of the total votes cast in that section.
The ballot must take place at such a time which means the results can be declared at the annual party conference or at a special leadership election conference.
If any candidate receives a majority of votes, he/she is declared the leader.
If not, the last place contender drops out and the second preference reallocated, until someone passes the 50 per cent figure.