In an afternoon of high drama in Westminster, politics.co.uk tracks those Labour MPs publicly joining Hoon and Hewitt's calls for a secret ballot.
The afternoon will provide the most obvious sign of whether the letter, from former Cabinet members Geoff Hoon and Patricia Hewitt, will trigger a rebellion big enough to oust Gordon Brown.
Below you can see which Labour MPs have publicly said they back the ballot demand:
Patricia Hewitt - Blairite former health secretary who helped bring in the smoking ban. Her career in the health department was marred by a motion of no confidence led by the Conservatives which she survived by 66 votes. She resigned from her post in June 2007, coinciding with the appointment of Gordon Brown as prime minister. Last year, following the expenses scandal, she announced she would be stepping down from the House of Commons.
Geoff Hoon - Ashfield (Nottinghamshire) MP since 1992, and has served as defence secretary, transport secretary, leader of the House of Commons and Chief Whip. Has courted controversy on a few occasions - as defence secretary in 2003 he stated in a BBC interview that Britain would be willing to use nuclear weapons against Iraq "in the right circumstances". He also came under fire for his expense claims last year, when it emerged he had 'flipped' his constituency home while renting out a London property and living in a state-owned rent-free apartment.
He is not renowned for rebelling against the party line but twice attempted a coup against Tony Blair. He resigned from the Cabinet last June, stating he wanted to spend more time with his family.
Backbench MP John Mann told the BBC: "Geoff Hoon is a serial plotter against prime ministers. It's about their egos, their attempt to position themselves, especially Hoon. He didn't get the Europe job [of EU high commissioner which went instead to Baroness Ashton], it's sour grapes."
Frank Field - Infamous left-winger of the old-school variety, although serious concerns with imigration have seen him make cross-party alliances. Serial rebel backbencher. He has represented Birkenhead since 1979 and is popular with his constituents, with a majority of 12,934 - the 16th highest majority of all MPs. A serial rebel against Brown, the fight between the two men became brutal and personal during the 10p tax debacle.
Charles Clarke - Blairite Norwich South MP since 1997, who left his position as home secretary in Tony Blair's Cabinet in 2006 after intense media interest in the deportation of foreign prisoners. This week he strongly suggested Brown should go if Labour was to have any hopes of electoral success.
In an email to colleagues, he said: "All the evidence suggests that Brown's leadership reduces Labour support, that alternative leaders would improve our ratings, and that an election determined by voters' answers to the question 'Do you want Gordon Brown to be Prime Minister for the next five years?' would further shrink Labour support."
Barry Sheerman - Chairman of the children, schools and families committee and persistent rebel. Considered one of the usual suspects. His relationship with children's secretary Ed Balls, a close Brown ally, fell to rock bottom late last year when he described him as a "bully".
Janet Anderson - The Rossendale and Darwen MP and former Casbinet member told the Lancashire Telegraph: "I very much welcome this proposal, and I think we should have a ballot as soon as possible to clear up the question marks over Gordon's leadership. There is no doubt that many of my constituents in Rossendale and Darwen are unhappy with it, especially women. We need to clear it up once and for all. There are concerns both among Labour MPs and party members and we need to get it cleared up."