Keir Starmer is expected to resign as British prime minister, pitching the party into a contest to choose its next leader and the next occupant of 10 Downing Street.
Here, the Press Association looks at what would happen next:
– How do candidates enter the contest?
Labour’s rules mean even getting onto the ballot can be a complex process.
First, candidates must secure the backing of 81 Labour MPs, 20 per cent of the party’s parliamentary strength.
They then need to receive nominations from either 5 per cent of constituency Labour parties, or three affiliated organisations (which must include two trade unions) which represent 5 per cent of affiliated membership.
Only after passing those two stages will candidates go before the party membership.
– Who could stand in a contest?
Andy Burnham’s decision to stand in the Makerfield by-election – giving up the Greater Manchester mayoralty to become an MP – suggests he believes he has the necessary backing from the parliamentary party to get onto the ballot.
Former British health secretary Wes Streeting has repeatedly insisted he has the 81 names required to mount a leadership bid, and has spent the weeks since his resignation setting out his stall.
Former Royal Marines officer Al Carns, who quit as armed forces minister in a row over defence funding and the treatment of Northern Ireland veterans, has also hinted he would seek to enter a leadership race, although it is unclear whether the MP – who was only elected in 2024 – has the supporters required to get on the ballot paper.
– How would a Labour leadership contest work?
After a leadership hopeful nails down 81 backers in the Commons and support from constituency parties and trade unions, candidates are then put to a vote among party members, who rank them in order of preference.
A contender is declared the winner if they get more than 50 per cent of first preferences, and this usually happens through a process of elimination during rounds of voting, the timetable for which is set by Labour’s ruling National Executive Committee (NEC).
– How long will this take?
Labour’s NEC will set the timetable for the whole election, including how long potential candidates have to secure nominations and how long members will be able to vote.
The party’s last leadership contest, in 2020, ran for around three months, with nominations opening in early January and the result announced in early April.
NEC sources have previously suggested there is little appetite for a long contest, with a maximum of two months being floated.
But Burnham’s supporters have suggested a “transition” period that would see the new leader take over in September, which would mean a longer contest.
– What would a coronation look like?
Andy Burnham’s significant support among the parliamentary party could mean he ends up as the only candidate.
With no need for a membership ballot, that scenario could see the contest wrapped up within a matter of weeks or even days.
But given Burnham’s apparent preference for a three-month “transition” period to allow him to prepare for government, it is unclear what would happen next in the event he was the only candidate.
Insisting on a transition would leave Starmer as a lame duck over a period that includes a Nato summit where countries are expected to set out plans to increase defence spending, and a UK-EU summit in Brussels seen as crucial to Labour’s attempts to reset relations with the bloc.
There is some precedent for such a coronation, however. Gordon Brown was the only candidate to receive enough nominations in Labour’s 2007 leadership election more than a month before Tony Blair formally stepped down as prime minister.
In that period, Brown still attended leadership hustings around the country to set out his ideas and attempt to win over the public.