The prime minister's political future hangs in the balance, after his Health Secretary Wes Streeting resigned from government.
Although he has not yet triggered a leadership contest, Streeting's allies previously told the BBC they expected him to challenge Sir Keir Starmer as soon as Thursday.
It comes after a handful of ministers and nearly 90 MPs called on the prime minister to go, if not immediately then in the near future.
However, there is not a consensus among Labour MPs about who they would want to replace Sir Keir as leader of the Labour Party and of the country.
Downing Street has said Sir Keir will fight any leadership challenge mounted against him.
Although no one else has publicly said they would stand, here are some potential contenders.
Wes Streeting has been health secretary since Labour came to power in 2024 and shadowed the position in opposition for three years before that.
He was first elected to Parliament in 2015 having previously served as president of the National Union of Students and a London councillor.
In his 2023 memoir, he wrote about growing up in a council flat in London's East End, visiting his bank robber grandfather in jail and growing up as a gay Christian.
The health secretary 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.
He has previously been open about his leadership ambitions and has plenty of support from Labour MPs, particularly those on the centre and the right of the party.
His allies in the cabinet include Business Secretary Peter Kyle and Science Secretary Liz Kendall.
His potential status as the 'right-wing' candidate could make him unpopular with party members, who tend to be to the left of the parliamentary party.
Andy Burnham has strong support from Labour MPs and polls suggest he is the most popular Labour politician with voters.
He can also point to a long track record of governing, having served as Greater Manchester mayor for almost a decade, earning him the nickname "the King of the North".
Burnham has made no secret of his ambition for the top job.
There is one major obstacle blocking his route to No 10 - he is not currently an MP. That is something his allies are hoping can be rectified quickly.
He did apply to be Labour's candidate in the Gorton and Denton by-election earlier this year but was blocked by Sir Keir's allies on the party's ruling body.
If Burnham does return to Parliament, it will be his second stint in Westminster.
Between 2001 and 2017, he was MP for Leigh - and during that time held senior government roles including in the health and culture departments.
The 52-year-old has twice stood to lead his party - in 2010 when he lost to Ed Miliband and in 2015 when he came second to Jeremy Corbyn.
Much of his support in Parliament comes from the left of the party and MPs in the North West.
Deputy Labour leader Lucy Powell and Culture Secretary Lisa Nandy are both allies and would be likely to support Burnham if he were able to enter the race.
Angela Rayner was, until last year, the deputy prime minister and the most powerful woman in British politics.
It has been a remarkable journey for the woman who grew up in poverty and left school at 16 without any qualifications.
Through her job as a care worker she got involved in the trade union Unison, which became her launchpad into a political career.
In 2015, she was elected in the Greater Manchester constituency of Ashton-under-Lyne and rose quickly in Westminster, serving in Jeremy Corbyn's shadow cabinet.
In government, she took on the role of housing secretary and was tasked with rapidly increasing housebuilding and delivering an overhaul of renters' rights.
In 2025 she dramatically resigned, after admitting she had not paid enough tax on the purchase of a new home.
Like the other two main contenders, Rayner has strong support among Labour MPs, although as a Greater Manchester politician on the left, much of her base overlaps with Burnham's.
She has now settled £40,000 of unpaid stamp duty, following a HMRC investigation, which was previously seen as an obstacle to her standing.
With question marks and concerns hanging over the three main contenders, it is possible an unexpected candidate emerges.
Some Labour MPs have discussed former leader and current Energy Secretary Ed Miliband making a return.
He dismissed the suggestion in November telling the BBC: "I've got the T-shirt - that chapter's closed."
Home Secretary Shabana Mahmood has also been mentioned as a possible contender. However, her immigration changes have been controversial with Labour MPs and she could struggle to get support from party members.
Defence Minister and Royal Marine veteran Al Carns has long been seen as having leadership ambitions and this week has set out his diagnosis of Labour's problems in the left-wing New Statesman magazine.
His allies have told the BBC they expect him to stand in a contest if one is triggered.
Under the party rules, there is nothing to stop Sir Keir Starmer standing in a leadership race - and on Monday he told journalists he would do exactly that, if a contest emerged.
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