The second Tory leadership race in three months is under way after the extraordinary resignation of Liz Truss.

The outgoing prime minister was forced from office 44 days into her tenure after a seismic few weeks in Westminster in which her tax-slashing mini-budget crashed and burnt.

Ms Truss’s resignation, which makes her the shortest-serving prime minister in modern British history, followed a raft of humiliating U-turns, the loss of two of her most senior Cabinet ministers and open revolt by Tory MPs.

All eyes are now on who could replace her – with speculation mounting that Boris Johnson could launch a comeback to frontline politics, six weeks after he was officially ousted from the top job.

He is on holiday in the Dominican Republic.

Party rules for the leadership contest mean hopefuls need the backing of at least 100 Tory MPs by Monday afternoon to stay in the race.

This means the maximum number of people able to stand is three. If three candidates get 100 backers, there will be a vote by MPs, with the winning two put forward to the party membership.

Sir Graham Brady, chairman of the powerful 1922 Committee of backbenchers, said: “We fixed a high threshold but a threshold that should be achievable by any serious candidate who has a prospect of going through.”

Who are the runners and riders?

Tory MPs are scrambling to find a replacement who can unite the party and turn around its fortunes after a series of dire polls predicted an electoral wipe-out.

The new inhabitant of No 10 will face a daunting set of challenges – from the cost of living crisis and rising energy bills to resolving the Northern Ireland protocol row, and war in Ukraine.

Jeremy Hunt, the chancellor, has ruled himself out of the running.

Rishi Sunak, the former chancellor and Tory leadership finalist, has signalled he is “very, very up for the job”, according to Sky’s deputy political editor Sam Coates.

Penny Mordaunt, the Commons leader, has also made it “clear in her public appearances that she’s up for the job”.

Suella Braverman, who resigned as home secretary on Wednesday, was highly critical of Ms Truss when she stepped down – an indication, her allies believe, of her intention to run.

Coates says: “The question now is who will stand aside and who will survive in an intense, week-long leadership contest.”

Nominations opened on Thursday and will close at 2pm on Monday. The new leader will be chosen by Friday 28 October.

The final two candidates will take part in a hustings event organised with news broadcasters, before an online vote by party members.

Should Tory MPs coalesce around one candidate, however, the contest will be over on Monday.

‘Bring back Boris’

On Thursday night, momentum appeared to be swinging behind Boris Johnson amid reports he will throw his hat into the ring.

Nadine Dorries, the former culture secretary and a staunch Johnson ally, told Sky News she is confident he will meet the threshold of 100 MPs.

“There is only one MP who has the mandate of the British public, who won a general election only three years ago with an 80-seat majority, and that was Boris Johnson,” Ms Dorries said.

“He is a known winner and that is certainly who I’m putting my name against because I want us to win the general election. Having a winner in place is what the party needs to survive.”

Although multiple Tory MPs have expressed their support for a Johnson comeback, he remains a divisive figure.

Senior backbencher Sir Roger Gale MP tweeted to remind people that the former prime minister, who resigned in a mire of sleaze, was still under investigation by the Privileges Committee for potentially misleading the House over partygate.

If found guilty, Mr Johnson could face recall proceedings that would leave him battling for his seat in the Commons if he receives a suspension of 10 days or more.

Sir Roger told Times Radio that, if Mr Johnson is voted back in as prime minister, he would give up the Conservative Party whip and stand as an independent.

Read more:
Resignations, reversals and rebellion – the 44 days of Liz Truss’s premiership
The divided Tories won’t find it easy choosing a new PM

Could Johnson reach 100 nominations?

There are 357 Conservative MPs, 148 of whom voted against Mr Johnson in a confidence vote a few months ago.

If they refuse to back him again, that leaves 209 MPs to potentially back him.

Liz Truss had 113 nominations this summer, so these are all up for grabs.

Brendan Clark-Smith, the Conservative MP for Bassetlaw, is among the group calling to “bring back Boris”.

A big problem with a Johnson candidacy is his falling popularity with the public – even if he still rides high with the Tory membership.

Polling for the Conservatives was already dropping during Mr Johnson’s premiership as it became beset with scandals, including the former prime minister breaking his own lockdown laws.

Mr Johnson was forced to announce his resignation on 7 July as Cabinet allies turned on him with a series of resignations.

The final straw was questions about his judgment over Chris Pincher, the Tory whip who was the centre of drunken groping allegations. That came on top of Mr Johnson’s attempts to change the rules to prevent the suspension of the Conservative MP Owen Paterson after he broke lobbying rules.

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Ms Truss officially took over from Mr Johnson on 6 September, with members favouring her tax-slashing plan for growth over Mr Sunak’s more conservative fiscal policies.

But just two weeks into the job, her disastrous mini-budget unleashed chaos in the financial markets, leading to the sacking of Kwasi Kwarteng as chancellor and a humiliating abandonment of the very economic policies that brought her into office.

Many MPs have voiced their support for Mr Sunak, who had warned Ms Truss that her economic policies were “immoral” and campaigned for fiscal responsibility during the last leadership race.

Jonathan Djanogly and Mark Garnier both tweeted their support for him late on Thursday night.

Richard Holden, another Tory MP, said that in the “difficult economic times, the party and the country needs a PM who has got the economic experience to deliver real stability over the next few years and get the ship of state back on an even keel – and that person is Mr Sunak”.

‘Last chance saloon’

A vicious leadership contest would further split an already divided party that is soon to have its third prime minister in the space of a few months – and many Tory MPs are calling for colleagues to unite behind the next leader, whoever that may be.

Justin Tomlinson said the leadership contest is the “last-chance saloon” for the party to maintain credibility, while former cabinet minister Robert Jenrick warned the Tories face “extinction…if we get this wrong”.

Opposition parties say that, whoever is handed the keys to Downing Street, a general election must be called immediately.

Labour leader Sir Keir Starmer said the Conservative Party has “shown it no longer has a mandate to govern”, adding that British people “deserve so much better than this revolving door of chaos”.