Labour leader Jeremy Corbyn has vowed to fight any challenge for his job but here are some of the candidates to replace him if he is forced out:
1. Dan Jarvis
The former paratrooper has widely been talked of as a future Labour leader, with a back story including service in Kosovo, Northern Ireland, Afghanistan and Iraq that many think could inspire voters.
Relatively inexperienced, he has never served in the shadow cabinet and has worked from Labour's backbenches since Corbyn's election as leader.
He has, since 2011, been the MP for Barnsley Central - the South Yorkshire town where 68% of voters backed Brexit and a former mining area where Labour is losing working-class support to Ukip.
2. Angela Eagle
The shadow business secretary has impressed observers when standing in for Corbyn at Prime Minister's Questions when David Cameron has been away and George Osborne has filled in.
An experienced operator, Eagle served as a junior minister in the Treasury, Home Office and Department for Work and Pensions during much of Labour's time in government in the 1990s and 2000s.
A sharp-witted Commons performer, her rise to prominence picked up pace during her four years as shadow Commons leader under Ed Miliband - a position that will have given her good connections among the party's MPs.
3. David Miliband
Many thought Labour could have won the last general election had the elder Miliband brother been in charge instead of thousands of miles away heading up the International Rescue Committee in New York.
The former foreign secretary is seen as an arch-Blairite, which could appeal to some of those calling for Corbyn's head, but it could be difficult for him to win a leadership election among the party's more left-wing membership.
That said, his extensive diplomatic experience, global connections and centrist politics could give him an unassailable advantage as Britain heads into years of potential uncertainty following the vote to leave the EU.
He would have to be re-elected as an MP, with the difficult by-election in Batley and Spen following the horrific killing of Jo Cox presenting the only potential route back to the Commons at present.
4. Lisa Nandy
The shadow energy secretary insisted at the start of the year that she did not want to be Labour leader but is seen by some as a potential "soft left" unity candidate.
The Manchester-born MP has, since 2010, represented Wigan, another former coal mining town where nearly 64% of voters backed Brexit, and therefore could be seen as another candidate who understands working-class "old Labour" voters.
In May, she helped launched the Labour Together group aimed at unifying grassroots supporters, MPs and councillors with a programme attempting to bridge the gap between the Corbynite left and Blairite right of the party.
5. Owen Smith
Another 2010 intake MP seen as a potential "soft left" candidate, the Welshman has stated clearly that he would like to be Labour leader, saying the job would be "an incredible honour and privilege".
The impressive Commons performer has shone as shadow work and pensions secretary since September, and from then on the Government has been forced into U-turns on cuts to tax credits and disability benefits.
He could attempt to claim those as his victories in any leadership campaign, alongside his "real world" experience of five years in the biotechnology and pharmaceuticals industry.
6. Chuka Umunna
Seen as a potential winner of last summer's leadership race before dropping out, citing the added pressure and scrutiny, the former shadow business secretary is a favourite of the party's centrist MPs.
A polished media performer, the former employment lawyer's background as the son of a Nigerian father and English-Irish mother gives him a different perspective on immigration - seen as the issue which swung the referendum for Brexit.
But the Streatham MP may be seen as simply another "London metropolitan elite" candidate who will turn off working-class voters in the North.
7. Tom Watson
The deputy leader would take over as interim leader were Corbyn to resign but it is unclear whether he would seek to make the position permanent.
The experienced operator is seen as the party's ultimate "fixer" but faced criticism over his claims that Tory former home secretary Leon Brittan was a rapist, allegations which were investigated and dropped by the Crown Prosecution Service.