Angela Eagle is set to formally launch her bid to oust Jeremy Corbyn as Labour leader following a turbulent few weeks for the opposition and attempted peace talks breaking down between Tom Watson and the union leaders.
But how would a potential leadership contest actually work? Here's a brief summary:
A secret ballot of the Parliamentary Labour Party showed the vast majority - 172 against the leader, compared with just 40 backing him - had lost confidence in the veteran left-winger. While the vote carries no formal weight, it illustrated how little support there was for Corbyn among his MPs.
Any Labour MP wanting to challenge Corbyn will require the support of at least 20% of their parliamentary colleagues in Westminster and the European Parliament - at current levels, with 230 MPs and 20 MEPs, that equates to 50 signatures.
Those MPs would have to signal their support for a challenger by writing to the party's general secretary.
Should the 20% figure be reached, a formal contest would then take place at the Labour Party's autumn conference at the end of September.
Any other MPs wanting to stand for the leadership would also need to reach the 20% figure in order to get on the ballot.
Corbyn has insisted he will fight any challenge and his team claims legal advice indicates that under the party's rules he will automatically be on the ballot paper. But critics of the leader interpret the rules as meaning he too will have to secure nominations from MPs - something that is unlikely to happen. The party's National Executive Committee is expected to make a decision on the exact rules, but there is a possibility it could end up facing a challenge in court.
Unite general secretary Len McCluskey has warned "any attempts to keep Jeremy Corbyn, elected just 10 months ago with an enormous mandate, off the ballot paper by legal means risks a lasting division in the party".
Former leader Lord Kinnock, who faced a leadership challenge from Tony Benn in 1988 - which was backed by Corbyn - has insisted that the incumbent would require the support of MPs to stand.
Once a line-up of candidates is finalised, the election would then take place on a one-member-one-vote basis with members, affiliates and registered supporters taking part.
If there were more than two candidates, voters would rank their choices and the winner would be the first to secure more than 50% of the vote. If nobody did so in the first round of counting, the last placed candidate would have their votes reallocated, and so on, until a winner emerges.
However, when Corbyn was elected leader, he secured 59.5% of the vote in the first round after a surge in support from newly registered activists. He topped the ballot among party members, trade unionists and new supporters. That means any challenger would face an uphill battle to dethrone the current Labour leader - if he makes it on to the ballot. Labour Party membership reportedly rose from about 200,000 in May 2015 to almost 400,000 in January of 2016, with much of that surge attributed to Corbyn.