As Labour faces a potentially devastating defeat at the general election, here is a look at who could be in the race to replace Jeremy Corbyn as party leader.
The shadow chancellor played a prominent role throughout the general election campaign, but has appeared to have ruled himself out as a future leader.
Last night, Mr McDonnell said he will not serve "either as a temporary or a permanent" leader of the Labour Party if Mr Corbyn were to resign.
Back in October, Mr McDonnell said he "can't see" how he or close ally Mr Corbyn could continue to lead the party if they failed to win power after the next general election.
The shadow foreign secretary has deputised for Mr Corbyn in PMQs and has represented the Labour Party on various overseas visits.
Ms Thornberry, who campaigned for Remain in the 2016 Brexit referendum, joined the party when she was 17 and was motivated by her experiences being raised by her mother, a single parent living on a council estate, according to her website.
She was first elected as MP for Islington South and Finsbury on 5 May 2005.
The shadow Brexit secretary was a human rights lawyer before becoming an MP, and co-founded Doughty Street Chambers in 1990.
He worked as human rights adviser to the Policing Board in Northern Ireland, monitoring compliance of the Police Service of Northern Ireland (PSNI) with the Human Rights Act, and in 2008 he was appointed Director of Public Prosecutions (DPP) and Head of the Crown Prosecution Service for England and Wales.
Mr Starmer was elected as Labour MP for Holborn & St Pancras in May 2015.
Mr McDonnell named shadow education secretary Angela Rayner as a possible successor to Mr Corbyn in an interview with former Labour spin doctor Alastair Campbell for GQ magazine in October, saying whoever comes after Mr Corbyn "has got to be a woman".
Ms Rayner was brought up on a council estate and left her local comprehensive at 16 with no qualifications and pregnant, after being told she would "never amount to anything", according to her website. It adds that in 2015 she became the first woman MP in the 180-year history of her Ashton-under-Lyne constituency.
Her web page also says she rose through the ranks of the trade union movement to become the most senior elected official of Unison in the North West.
The shadow business secretary grew up by Old Trafford football ground in Manchester and began her working life serving at the counter of a pawn shop, according to her website.
She has also worked in call centres, a furniture factory, and as a postwoman before eventually studying to become a solicitor, her online biography adds, while she describes herself as a "proud Socialist" in her Twitter profile.
In 2015 she was elected as MP for Salford and Eccles.