Angela Eagle has announced that she will launch her bid to oust Jeremy Corbyn as Labour leader on Monday.
Following the turbulence which has hit the Labour Party in the weeks since the EU referendum, a leadership contest looks set to take place, but how do they actually work and what are they?
Who can stand?
Any Labour MP wanting to challenge the current leader will need the support of at least 20% of Labour MPs and MEPs. This equates to at least 50 signatures. Support is demonstrated for a challenger by writing to the party's general secretary.
The 20% is needed for a candidate to get on the ballot.
How does the vote work?
Once a line up of candidates is finalised, the election would then take place. Members, affiliates, and registered supporters all get one vote.
Voters rank their choices and the winner is the first to get more than 50% of the vote. If nobody manages this in the first round of voting, the last placed candidate would have their votes reallocated. This would carry on until the 50% threshold is reached.
Does this mean Jeremy Corbyn will stand down?
Mr Corbyn has insisted he will fight any challenge.
So Jeremy Corbyn will automatically be in the contest then?
Mr Corbyn's his team claims legal advice indicates that under Labour Party rules he will automatically be on the ballot paper. However, others say he will have to secure nominations from MPs and MEPs - something that he is likely to struggle with.
The party's National Executive Committee is expected to make a decision on what happens, but there is a possibility it could end up facing a challenge in court.
Is Jeremy Corbyn likely to win again?
When Jeremy Corbyn was elected leader, he secured 59.5% of the vote in the first round after a surge in support from newly registered activists.
Labour Party membership reportedly rose from around 200,000 in May 2015 to almost 400,000 in January 2016 with much of the surge attributed to Mr Corbyn.
What do high profile figures think?
Former leader Lord Kinnock has insisted that Mr Corbyn would require the support of 20% of MPs and MEPs to stand.