A new leadership race will take place within a week, Liz Truss said in her resignation speech.
He will be the fifth Conservative prime minister in just over six years – and the third in this legislature.
But who could be the next leader? Here are the main runners and runners.
Sunak has proven to be something of a prophet of government demise, as many of the predictions he made during this summer’s leadership about Truss’s economic plan have come true.
The former Chancellor of the Exchequer (Finance Minister) has warned that Truss’ unfunded tax cuts will lead to a run on sterling, panic in the bond market and concern at the International Monetary Fund. Perhaps he would even have been surprised by how quickly he was right.
Sunak has experience of battling the economic crisis, having guided the UK through the Covid-19 pandemic. He is also popular among MPs, having secured more votes in Parliament than Truss before the choice between the final candidates was put to the members, and only narrowly losing in the final vote.
The confidence he has among MPs – and the vindication of his predictions – could make him the next most likely set of hands to steer the ship.
The leader of the House of Commons may have had a dress rehearsal to be Prime Minister this week, after replacing an absent Liz Truss during a debate.
“The Prime Minister is not under a desk,” Mordaunt confirmed on Tuesday – in a performance that seemed as much to show up as to help the Prime Minister.
Mordaunt came third in the last leadership election, narrowly missing out on being introduced to the membership – among whom she was expected to do well, in part due to her military credentials. Mordaunt is a Royal Navy reservist.
Like Sunak, she belongs to the more moderate wing of the party. There have even been talks among MPs about forming a ‘dream team’ ticket, although that has yet to materialize – and it’s unclear whether Sunak would be content to be chancellor again.
It’s a sign of the latter-day mess of Truss’s government that she elevated Grant Shapps to Home Secretary – although she didn’t offer him a ministerial role of any kind when he was first taking office.
Shapps served as transport secretary under Boris Johnson. He stood to succeed him in the previous leadership election – only to drop out of the race three days later after failing to secure the 20 MP votes required to advance to the next round.
Badenoch came fourth in this summer’s leadership election – but has still been ranked by pollsters as one of the Tory base’s favorites.
One of the youngest MPs in the running, Badenoch quickly won the approval of longtime Tory top Michael Gove, who hailed her as the party’s “outstanding talent”.
Badenoch is on the right of the Conservative Party – and in his previous leadership bid suggested the government’s climate targets could prove too costly.
Just a few months ago, Johnson held a comfortable majority in Parliament and even claimed he was even considering his third term – to widespread derision. Despite being plagued by a seemingly endless series of scandals, Labor was still trailing in the polls – and Johnson had no obvious competitor in the party.
In his final speech as Prime Minister outside 10 Downing Street, Johnson made one of his signature allusions to ancient history. He said he would ‘return to his plow’ like the Roman statesman Cincinnatus – suggesting a quieter life on the bench seats.
But that’s not how Cincinnatus lives its days. He was called back from his plow to return to Rome for a second term – this time as dictator.
Memories of ‘Partygate’, the protracted scandal that ultimately led to his downfall, may prove too fresh for MPs to recall Johnson.
But, as the Conservative party faces electoral oblivion, the man who won an 80-seat majority in 2019 could prove a tempting option for many MPs.