Patricia Hewitt and Geoff Hoon, the two former Labour ministers who are urging a secret ballot on Gordon Brown's leadership, are senior and highly experienced backbenchers.
Both Labour MPs have held a number of Cabinet positions
Patricia Hewitt served as secretary of state for trade and industry and health secretary under Tony Blair.
Regarded as a Blairite loyalist, she left the Cabinet in 2007 when Gordon Brown became prime minister.
At the time, she said she wanted to devote more time to her family, some of whom live in Australia.
Since leaving office, she has not been openly critical of Gordon Brown's performance although she was seen as being close to a number of female ministers - including Jacqui Smith and Caroline Flint - who resigned from the Cabinet in 2008.
However, she spoke out against plans to phase out childcare vouchers for some working parents announced by Gordon Brown in his autumn conference speech - a move which led to the policy being scrapped.
Ms Hewitt, who is a non-executive director of BT, announced last summer that she would stand down as an MP at the next election.
Before being elected in the 1997 Labour landslide, she served as former leader Neil Kinnock's press secretary and co-wrote Mr Kinnock's famous conference speech attacking Militant.
Geoff Hoon is one of the most familiar political faces of the Blair-Brown years have served as a Cabinet minister under both prime ministers.
During his six year stint as defence secretary, the UK was involved in military action in Sierra Leone, Kosovo, Afghanistan and most controversially - Iraq.
He was criticised for his role in the decision to go to war in the 2003 invasion with critics saying he had been sidelined by Downing Street.
After the death of Dr David Kelly, the Ministry of Defence was criticised for aspects of the way it handled the process which led to the media naming him as the source of a controversial story on Iraq's weapons threat.
But Mr Hoon was largely spared from personal criticism over the issue in the Hutton report into Dr Kelly's death.
Mr Hoon is due to give evidence to the Chilcot Inquiry into UK involvement in the invasion in the next few weeks.
In 2005, Mr Hoon was moved by Mr Blair in 2005 to the less high-profile role of Leader of the House of Commons and then to a more junior role as Europe minister.
But he surprisingly returned to the Cabinet as Chief Whip when Gordon Brown - whom he was not seen as being close to - succeeded Tony Blair in 2007.
He later became transport secretary and it was thought he might become the UK's European Commissioner but did not land the job.
He left the Cabinet last summer in a wide-ranging reshuffle triggered by the resignation of a number of ministers in protest about Gordon Brown's leadership.
One Labour MP has accused Mr Hoon of being a "serial plotter" but Mr Hoon has insisted he has not spoken to any other MPs about the latest move against Mr Brown.
Mr Hoon, first elected to the Commons in 1992, is due to contest his Derbyshire seat again at the next election.