Sir Keir Starmer is fighting to stay on as prime minister, after dozens of MPs called for him to quit immediately, or set a timetable for his departure.
Health Secretary Wes Streeting, considered one of the main potential challengers for the PM's job, has resigned. He told Sir Keir: "It is now clear that you will not lead the Labour Party into the next general election."
Sir Keir has warned his ministers and MPs that a leadership contest could result in "chaos". More than 150 MPs have indicated support for Sir Keir or say it is not the right time for a leadership contest.
Labour performed badly in the biggest set of elections to be held since its landslide election victory in 2024. The polls were seen by many as a key test for Sir Keir's leadership.
Labour lost almost 1,500 councillors in local elections across England amid a surge for Nigel Farage's Reform UK, with the Greens also eating into the party's support.
It was also kicked out of power in Wales, where it had enjoyed political dominance for a century. Labour also won just 17 of the 129 seats in the Scottish Parliament, its worst ever result at a Holyrood election.
Leaving his post on Thursday, Streeting published a letter to the PM, in which he said: "Where we need vision, we have a vacuum. Where we need direction, we have drift." His supporters expect him to challenge Sir Keir for the Labour Party leadership.
Prior to this, Jess Phillips resigned as safeguarding minister, telling Sir Keir that "deeds, not words matter". Communities Minister Miatta Fahnbulleh, Victims Minister Alex Davies-Jones and Health Minister Zubir Ahmed have also resigned.
Home Secretary Shabana Mahmood is among those calling for Sir Keir to set a date for leaving.
The BBC is aware of nearly 90 Labour MPs who have called on Starmer to leave his post, or set out a timetable for his resignation.
However, more than 150 have indicated support for the PM, or say it is not the right time for a leadership contest.
Chancellor Rachel Reeves warned against a leadership contest "at a time when there is conflict in the world and also at a time when our plan to grow the economy is starting to bear fruit".
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 have 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 as their number one 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, known as 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.
Health Secretary Wes Streeting, Greater Manchester Mayor Andy Burnham and former deputy prime minister Angela Rayner are thought to be the main potential challengers to Sir Keir.
Wes Streeting
The health secretary backed the prime minister following Labour's election losses, but did not explicitly rule out standing in any contest.
Some who would like to see him replace Sir Keir think this might be his very best chance, before Andy Burnham can get back to Westminster.
Streeting has been linked to Peter Mandelson, whose appointment as British ambassador to the US - despite his links to convicted sex offender Jeffrey Epstein - led to questions being raised about Sir Keir's judgement.
After days of intense speculation that he was preparing a bid to replace him, Streeting announced his resignation at Thursday lunchtime.
In a letter to the PM, he said that people are "increasingly losing faith" in the Labour Party's responsibility of offering hope that Britain's best days lie ahead.
Streeting said the debate about what comes next needs "the best possible field of candidates".
Andy Burnham
Although he has made little secret of his leadership ambitions, Burnham cannot enter a contest without first becoming an MP.
He was blocked from doing so earlier this year by the NEC.
Refusing to allow Burnham to stand in the Gorton and Denton by-election, it said the cost to the party of a mayoral election to replace him would be "disproportionate".
The mayor's supporters are hoping a leadership contest can be delayed until he is able to return to Parliament.
Angela Rayner
Rayner is favoured by some on the left of the party.
In her first public comments since the elections, Rayner 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.
Rayner resigned as Sir Keir's deputy prime minister in September 2025 after it emerged she had failed to pay the correct amount of stamp duty on a £800,000 flat in Hove.
But on Thursday she said she has been "exonerated" by HRMC of the accusation she had "deliberately sought to avoid tax" in a statement.
In an interview with the Guardian, she did not rule out running in any Labour leadership race but said she would not "trigger" a contest.
Prime ministers hold office unless and until they resign. Sir Keir has told the cabinet that he will prove his "doubters" wrong and will not "walk away".
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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