Andy Burnham is expected to challenge Sir Keir Starmer for the Labour leadership after winning the Makerfield by-election.
Sir Keir has said he plans to fight any challenge and, if he wins, he would continue as prime minister.
However, if he loses, the winner would replace him in Downing Street without the need for a general election.
Labour performed badly in May's local elections, losing almost 1,500 councillors in England, while the party lost power in Wales and recorded its worst ever result for the Scottish Parliament.
More than 90 Labour MPs have publicly urged the PM to resign immediately or draw up an exit timetable. However, more than 150 MPs have either indicated support for Sir Keir or say it is not the right time for a leadership contest.
Discontent towards Sir Keir's leadership had been mounting before the elections, including over his decision to change direction on three major policies in a month after pressure from within his own party.
Sir Keir's decision to appoint Lord Mandelson as British ambassador to the US also led to questions about his judgement and the wider Downing Street operation.
Lord Mandelson was sacked after new information came to light about the depth of his relationship with the late convicted sex offender Jeffrey Epstein.
MPs unhappy with Sir Keir remaining in post could try to force a leadership election.
This would require 20% of the party's MPs to back a replacement candidate. There are 403 Labour MPs, so the support of 81 would be needed.
Once a Labour leadership election is triggered, other candidates could join the contest if they also had 81 backers.
Sir Keir would not be required to gather support as he would be automatically on the ballot paper if he chose to contest the leadership election.
He would continue as prime minister during any contest.
No Labour prime minister has ever faced a formal leadership challenge from their MPs.
Party members and affiliated trade union supporters vote by ranking the candidates in order of preference – putting a one next to their favourite, two for their second choice and so on.
If one candidate receives more than 50% of first preferences then they will be declared the winner.
If not, then the candidate who finishes bottom will be eliminated and the voters who put them first will see their vote moved to their second choice.
This process of elimination continues until one candidate receives more than half of the votes.
The timetable for the leadership ballot would be decided by Labour's decision-making body, the National Executive Committee (NEC).
In 2020, candidates had time to secure their nominations before a ballot took place over a six-week period.
That leadership contest was triggered when Jeremy Corbyn announced he would stand down following the party's defeat in the 2019 general election.
Former Health Secretary Wes Streeting, newly elected MP Andy Burnham and former Deputy Prime Minister Angela Rayner are considered the main potential challengers to Sir Keir.
Wes Streeting
After stepping down as health secretary, Streeting told the prime minister he had "lost confidence" in him.
He has confirmed that he would enter any potential Labour leadership contest.
Streeting is seen as the cabinet's best communicator and can point to a fall in NHS waiting lists as one of his achievements in government.
Andy Burnham
Burnham has argued Labour "needs to change if we are to regain people's trust".
His first stint as an MP ran from 2001 to 2017 when he represented Leigh. He served as a minister in the governments of Sir Tony Blair and Gordon Brown, but failed to win two Labour leadership elections when the party was in opposition.
Burnham left Westminster to stand for election as the mayor of Greater Manchester, which he won in 2021. He was re-elected in 2024.
He won the Makerfield by-election on June 18 which has paved the way for him to challenge Sir Keir as party leader.
Angela Rayner
Rayner is favoured by some on the left of the party.
She has called for Labour to offer regional mayors more economic powers and raise the minimum wage. "We are in danger of becoming a party of the well-off, not working people," she said.
In 2025, she resigned as deputy prime minister, after admitting she had not paid enough tax when buying a new home. However, she has settled £40,000 of unpaid stamp duty and said she has been "exonerated" by HRMC of the accusation that she had "deliberately sought to avoid tax".
Prime ministers hold office unless and until they resign. Sir Keir has said a leadership contest would be a "bad thing" for the country and said that he intends to fight any challenge.
However, if this changed, he could choose to stay in post during a transition period before handing over to a successor, who would be both Labour leader and prime minister.
If Sir Keir chose to resign with immediate effect, a member of the cabinet would replace him in a caretaker capacity, under Labour Party rules.
This would involve a decision by the cabinet, who would consult with the NEC.
Deputy Prime Minster David Lammy would not necessarily fill the caretaker role.
A leadership contest would follow.
In this scenario, each potential challenger would need support from 20% of the party's MPs and to be supported by either 5% of constituency Labour parties (CLPs), or at least three affiliated organisations - of which two must be affiliated trade unions.
Whoever won a Labour leadership election would automatically be the prime minister without the need for a general election.
However the PM must have the "confidence" of the House of Commons to govern, which means they must be supported by a majority of MPs.
If the leader of the opposition, currently the Conservative's Kemi Badenoch, introduces a motion of no confidence, the government is expected to provide parliamentary time for a debate and a vote.
To succeed, the motion needs just one more vote in favour than against.
If the government loses the vote, a general election is usually called.
Important caveats to note are that 403 of Westminster's 650 MPs are Labour.
Several Labour MPs would therefore have to support any motion for it to succeed, which is very unlikely.
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