Ousted Prime Minister Boris Johnson is being touted for a sensational return to No10 as the race to succeed Liz Truss begins.
Tory chiefs have fired the starting gun on the second leadership contest in a matter of months following the dramatic collapse of Ms Truss's premiership in just six weeks.
A new Prime Minister could be announced as early as Monday night or Tuesday morning if only one candidate emerges - or by Friday if there is a contest between two rivals.
Supporters of Mr Johnson immediately started calling for the scandal-ridden former PM to make an extraordinary comeback, only months after he was forced to quit by his own MPs.
Business Secretary Jacob Rees-Mogg was among MPs tweeting a new graphic with the slogan: 'Boris or Bust'.
Mr Johnson could find himself up against his bitter rival - and his former Chancellor - Rishi Sunak, as well as Commons Leader Penny Mordaunt who is gathering early support.
More than 40 MPs have come out for Mr Sunak, with around 35 publicly backing Mr Johnson and some 15 supporting Ms Mordaunt.
Labour demanded an immediate general election, with Shadow Chancellor Rachel Reeves saying No10 and No11 are not a "short term AirBnB".
Critics said there would be a "wave of revulsion" if the controversial former Tory leader returned to Downing Street.
A return to power for Mr Johnson would be hugely divisive in his own party, with some Tory MPs saying they would quit rather than serve under his leadership.
Mr Johnson still faces a Commons probe over claims he lied to Parliament about lockdown boozing in Downing Street, which could potentially see him expelled as an MP.
A Cabinet Minister told Sky News that Mr Johnson could get the backing of more than 100 MPs - the threshold needed to get on the ballot paper.
"Even people who resigned from his government were on the terrace yesterday telling colleagues they would now back him and members definitely will," the unnamed Minister said.
Tim Montgomerie, founder of Tory bible Conservative Home website, told BBC Radio 4's Today that people should be taking rumours of a Boris Johnson leadership campaign "very seriously".
"I've changed my own opinion over the last 24 hours. I'm now hearing from two or three people I really respect who are usually very good at the numbers that he is not only likely to pass the 100 MPs threshold but could get close to 140."
Mr Johnson has been on holiday in the Caribbean - missing the ousting of his successor - but he is believed to be jetting back to the UK for a run for the top job.
His former press secretary Will Walden said he'd been told Mr Johnson was on his way back and "clearly he's taken soundings" from supporters.
But Mr Walden said he would be unlikely to run if he didn't think he could get the necessary 100 MP backers to get on the ballot paper.
He told Sky News: "The thing about Boris is that he absolutely hates to lose and I suspect that if he doesn't think he can get to the 100, or more importantly he can come second in this in the first vote as Rishi Sunak is clearly the frontrunner, then he won't run.
"If he does think he can then I'm sure he will because in his mind he's done nothing wrong."
A friend of the ex-PM told the Mirror they did not know if he was planning to stand but added: "You know what he says about the ball coming loose in the scrum.
“I think it would be in character if he did."
Staunch Johnson loyalist Nadine Dorries, the former culture secretary, said she had spoken to him since Ms Truss's resignation, hinting strongly that he was preparing to run.
"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," she told Sky News.
Mr Johnson has been privately urging Tory MPs to back him or risk electoral wipeout, according to reports.
“If the Tories are serious about winning in 2024 and want to stop a general election before then they need to revert to the guy with a mandate who is a seasoned campaigner," an ally told the Telegraph.
"They need someone to take the fight to Labour. There’s no point going to a yellow box junction without knowing how you are going to get out of it. Rishi should make contact and work out how the two of them can get back together.”
But Sir Roger Gale, a long-time critic of Mr Johnson, warned that he could be met with a wave of resignations by Tory MPs.
"I think that there would be people, indeed like myself, who would find ourselves in the awful position of having to resign the Conservative whip," he told Times Radio.
Veteran Tory Crispin Blunt said Mr Johnson is "not the character" to lead the Conservative Party at this time.
In a sign of the deep divisions, Conservative MP Tim Loughton publicly attacked Mr Rees-Mogg for tweeting the slogan 'Boris or Bust'.
He tweeted: "Jacob-how on earth can that slogan be remotely helpful to the Party given the strong possibility that the next PM will not be Boris?
"I would not use the tag line #BorisandBust and you really should think this through properly if you have any interest in party unity."
SNP Westminster leader Ian Blackford said people were horrified about the prospect of Mr Johnson returning to No10.
"There's real concern that the Tories might want to foist Boris Johnson back onto us again - I think there will be a wave of revulsion throughout Scotland."
Liberal Democrat Deputy Leader Daisy Cooper described him as "Britain's Berlusconi" and said the public would not forget his record.
She said: “The British public will never forget the way Boris Johnson repeatedly lied to them over his Covid lawbreaking, Partygate and the subsequent cover up proved he is unfit to govern this great country.
"From his illegal lockdown parties, to his Government's unlawful failure to protect care home residents during the pandemic, Boris Johnson's tenure as Prime Minister demonstrated that he simply has no regard for rules, or for the general public."
Lobby Akinnola, spokesperson for the Covid-19 Bereaved Families for Justice campaign said: "It’s devastating for bereaved families to think that anyone in the Conservative party is considering bringing Boris Johnson back as PM.
"This is the man who oversaw the deaths of 200,000 British citizens to Covid-19 on his watch. The man who partied whilst we had to say goodbye to our loved ones over a screen, and then spent six months months lying to us about it.
"The man who joked about ‘letting the bodies pile high’ while he refused to learn lessons from the first wave and let even more people die in the second."
But the Ukrainian Government appeared to throw its weight behind Mr Johnson.
The official Twitter account posted a meme based on the Netflix show Better Call Saul, with a photo of Boris Johnson's face, saying "Better Call Boris."
Keir Starmer said he was "rather surprised" by the now-deleted tweet but added: "As far as Ukraine is concerned, my absolute imperative is to make sure that we're all supporting Ukraine in the face of aggression from (Russian President Vladimir) Putin."
Ms Mordaunt is taking "soundings" from MPs about whether to stand, a source said.
"It's a testament to Penny's campaign over the summer how many colleagues have already come out asking her to stand," the ally said.
"At the moment there isn't a campaign but Penny has always been the candidate that can unite the party, deliver and beat Labour.
"At the moment she's been taking soundings from her colleagues and has been busy speaking to as many as she can."
Defence Secretary Ben Wallace is also in the frame, though he hasn't confirmed whether he will run. He decided not to stand in the summer leadership race.
Former Home Secretary Suella Braverman and Trade Secretary Kemi Badenoch are also potential contenders.
No one has publicly declared they will run yet but time is running out, with the deadline for nominations closing at 2pm on Monday.
Tory party chiefs have drawn up new rules demanding the candidates get the backing of 100 MPs to even make it onto the ballot paper, in a bid to whittle down the list quickly.
A maximum of three candidates can make the list, as there are only 357 Conservative MPs.
