More than a dozen Labour MPs are calling for leadership nomination papers to be sent out to all of the party's MPs, to see if there is enough support for an election. But who are the MPs leading the charge to challenge the prime minister?
The first minister to quit over the leadership issue. A former Catholic priest, he has said he could not go on backing Mr Brown in public while harbouring serious doubts about his leadership in private. The Scotland Office minister is close to other rebels Siobhain McDonagh and Joan Ryan but he has denied being part of a coordinated plot to oust Mr Brown. An MP for his home area of Greenock and Inverclyde since 2001, he rose rapidly through the junior ministerial ranks and was promoted last year by Mr Brown to the job of effectively running the Scotland Office, leaving Des Browne to focus on his other job of defence secretary.
A member of Labour's ruling National Executive Committee, she became a culture minister in Tony Blair's first government but was returned to the backbenchers in the 2001 reshuffle. She rarely rebels against the government, voting with it on contentious issues such as the Iraq war and counter-terrorism laws. When she was elected to Parliament in 1992 she became Rossendale and Darwen's first woman MP and Darwen's first Labour MP.
The MP for Mitcham and Morden caused a stir on Friday when she became the first member of government to break ranks and publicly call for a challenge to Gordon Brown's leadership. She was sacked as an assistant whip. A government spokesman suggested she had always been "anti-Gordon". She was the only member of government not to nominate Mr Brown for the party leadership after Tony Blair stood down. A long-term Labour activist, she made her first conference speech age 23 and was also one of the 1997 intake of MPs. Her sister Margaret was Labour's first female general secretary.
The former Home Office minister was sacked as Labour's vice chairman, and Mr Brown's envoy to Cyprus at the weekend when she joined the revolt. The MP for Enfield North, a former teacher, urged a "multiplicity of candidates" to come forward and trigger a "deep and far reaching debate". Among her tasks as a Home Office minister was handling the government's controversial plans to introduce ID cards. She was made Cyprus envoy when Mr Brown became party leader. Earlier this month she took part in negotiations alongside Europe minister Jim Murphy over reunifying the island.
The Slough MP, another of Labour's 1997 intake, she was initially appointed as a Parliamentary Private Secretary to Chris Smith, the then culture secretary and was later a Home Office minister. She had been a former chairman of the civil rights campaign group Liberty and was a primary school teacher before becoming an education lecturer. She joined calls for a leadership election at the weekend, urging senior ministers to show "courage" and telling the BBC: "I think we should give a chance to someone else to take over - I really do."
An MP since 1986, George Howarth was a minister in the 1997 Parliament - first in the Home Office and later as a junior minister in the Northern Ireland office - but left the government in the 2001 reshuffle. MP for Knowsley North and Sefton East, the former teacher and councillor is a close ally of Jack Straw. There were newspaper suggestions in July that he had been canvassing support for a possible move against Mr Brown, which he denied.
Along with Graham Stringer, Gordon Prentice was one of the few Labour MPs to publicly call for a change of leader back in July, following the party's Glasgow East by-election defeat. He urged Mr Brown to resign in Labour's "best interests". The MP for Pendle was one of the few MPs not to nominate Mr Brown to replace Tony Blair during last year's leadership election. He was critical of the decision to axe the lowest, 10p, tax band, calling it a "terrible mistake". He has also rebelled on issues like the Iraq war and renewing the UK's Trident nuclear weapons.
The Manchester Blakeley MP, a former leader of the city council who was elected as an MP in 1997, became a Cabinet Office minister shortly afterwards and a government whip in 2001, but returned to the backbenches in the 2002 reshuffle. As early as May, in the wake of Labour's defeat by the Tories in the Crewe and Nantwich by-election, he was saying the party needed a new leader to save it from "disaster" at the next election. He also rebelled against the government's plans to renew Trident.
The MP for Hayes and Harlingon, one of the most rebellious left wing MPs, was one of two to say they would stand against Gordon Brown for the party leadership after Tony Blair stood down last year, but failed to get the 44 nominations necessary, even after fellow challenger Michael Meacher stood aside. Elected in 1997, he had been Ken Livingstone's deputy on the Greater London Council where he was its chair of finance. A prominent supporter of the Stop the War coalition, he has rebelled on issues from ID cards, to limiting access to incapacity benefit and detaining foreign terrorist suspects without trial.
Normally loyal to the party leadership the Hyndburn MP was among the 10p tax rebels, tabling a Commons motion calling for action, before withdrawing it after "assurances" from the chancellor. He has been an MP since 1992, he was made a whip in 1995 and remained one throughout the 1997 Parliament. He returned to the backbenchers in the 2001 reshuffle.
Leader of the backbench 10p tax rebellion earlier this year, the Birkenhead MP later apologised to Mr Brown, saying he regretted allowing his campaign to "become personal". It followed a BBC interview in which he suggested Mr Brown was "unhappy" as PM and was prone to rages. Appointed as a welfare reform minister in Tony Blair's first government with a brief to "think the unthinkable," he resigned after reported rows with Mr Brown and Harriet Harman.
The MP for Liverpool Walton helped organise Tony Blair's campaign for the party leadership in 1994, but later became one of his most vocal backbench critics after resigning as defence minister in 2000. He opposed the Iraq war, tuition fees and was one of the Labour rebels who helped defeat Mr Blair's plans to hold terrorist suspects for up to 90 days without charge. He was one of the few Labour MPs not to nominate Gordon Brown as Mr Blair's successor in 2007.
The third person to leave the government over calls for a leadership election, Mr Gardiner - a special envoy on forestry for Mr Brown - accused the PM of "vacillation, loss of international credibility and timorous political manoeuvres". A former insurance adjuster and Cambridge researcher he won Brent North in 1997 from the sitting Tory MP and increased his majority in 2001, with the largest pro-Labour swing in the country. In 2002, he became Parliamentary Private Secretary to Home Office Minister Beverley Hughes and was later a junior minister at the Northern Ireland Office, Department for Trade and Industry and later at Defra - before being appointed special envoy in July 2007.