Geoff Hoon and Patricia Hewitt have led a call for a secret ballot of peers and MPs over Gordon Brown's leadership but Labour Party rules set out a number of tough conditions before an official contest is sparked.
At least 20 per cent of the Commons members of the Parliamentary Labour Party are needed to initiate a challenge.
That would mean 71 of Labour's MPs supporting such a move.
If that happened Mr Brown might decide to stand down voluntarily.
But if Labour's leader is prime minister at the time and refuses to resign then his opponents need to clear another hurdle - as an election can only proceed if it is requested by a majority vote at a party conference.
If these conditions are satisfied then an electoral college of unions, MPs and party members will vote on each candidate. Each group has a third of the vote and each member has one vote to cast.
Constituency parties and unions would normally ballot their members on the candidates in order to represent their views when they vote.
A series of postal ballots would then be held - in the 2007 deputy leadership contest the party used the alternative vote system.
Under this system voters rank each candidate in order of preference. The candidate who receives a majority of first-preference votes is elected. If no candidate gains a majority on first preferences, then the second-preference votes of the candidate who finished last on the first count are redistributed. This process is repeated until someone gets more than 50 per cent.
If Mr Brown chose to resign in the face of widespread dissent then the party would have to appoint an interim leader.
This could be the deputy leader but the final decision would be made by the Cabinet. Alternatively the current leader could remain as a caretaker leader until the conclusion of a contest.
If there is a vacancy to be filled then only 12.5 per cent of the PLP is needed to call for a leadership election.
And of course a resigning prime minister would be required to go to the Queen to inform her of his decision so she could ask somebody else to form a government in his place.Reuse content