Liz Truss kickstarts leadership race after ending chaotic 45 days as PM

At a lectern outside Downing Street during another tumultuous day, Truss admitted that she could not deliver the radical economic mandate on which she was elected by Conservative members.

She will be the shortest-serving UK prime minister ever, having presided over what has been one of the most politically turbulent and economically damaging periods in modern history.

Keir Starmer, the Labour leader, led calls for an immediate general election, arguing it was untenable for the Conservatives to appoint yet another premier without a fresh mandate. Instead, Tory grandees set a deadline of Friday 28 October for the new contest to be completed.

The rivals to succeed Truss immediately embarked on urgent campaigns to sign up at least 100 Tory MPs by Monday afternoon to get on the ballot paper, with Sunak quickly gathering momentum to become the early frontrunner.

Johnson was said by MPs to be considering another tilt at the job, despite quitting in disgrace in July following a series of scandals that left his personal integrity in tatters. Friends said he was planning to return early from a Caribbean holiday to gather support.

Tory insiders claimed that, although popular with party members, Johnson was a deeply divisive figure among MPs and would struggle to reach the required threshold for nominations.

Penny Mordaunt, the leader of the Commons who came third last time round, could also make the first cut unless the right of the party backs one of their own instead. If only one candidate reaches the 100-MP threshold on Monday, then they will automatically take over.

Foreign secretary James Cleverly, Tory heavyweight Michael Gove and centrist Tom Tugendhat have all said they will not stand.

Treasury sources said they were still working towards holding the next fiscal statement, which will set out spending plans, on 31 October. However, the new prime minister may delay that if they want to develop their own plans to steady the economy. The pound rose on the news that Truss was quitting.

Shortly after 1.30pm Truss stood before a lectern placed outside the No 10 door to give a statement that lasted just 90 seconds.

She and Brady, who chairs the 1922 Committee of Tory MPs, had agreed a timetable under which a successor would be chosen in just a week, Truss said, as opposed to the near-two months contest over the summer which saw her succeed Boris Johnson.

Candidates to replace Truss will need at least 100 nominations in order to stand, a hefty threshold set by the executive of the 1922 Committee of backbenchers in order to avoid a protracted contest. The threshold will mean a maximum of three candidates are likely to be nominated, and potentially only one or two.

Unless there is only one candidate who reaches the nominations threshold, there will be a vote from the membership, despite MPs expressing a strong desire to exclude members from the process.

Sunak received more than 140 votes in the last round, and his allies have sent a call out to backers to begin tweeting their endorsements from Thursday evening.

Mordaunt is also likely to be a strong candidate with Brexiter credentials; her last leadership campaign was blunted by attacks from Truss.

Two candidates from the right, Kemi Badenoch and the recently sacked home secretary Suella Braverman, were said to be holding talks about whether one of them could make a bid.

George Canning, until now the shortest-serving prime minister in British history, died in office after 119 days in August 1827.