Rishi Sunak has become the first Tory MP to secure the 100 nominations needed to enter the race to be the next leader of the party and prime minister.
The ex-chancellor is expected to launch a fresh bid for the top job after losing to Liz Truss over the summer.
Boris Johnson is flying back to the UK, as supporters rally behind a potential comeback by the ex-prime minister.
Commons leader Penny Mordaunt is so far the only contender to have officially launched a leadership campaign.
The hopefuls have until 14:00 BST on Monday to find 100 backers.
If any candidate reaches 158 nominations out of the 357 Tory MPs the race will be reduced to two candidates, as there will not be enough MPs left to support a third candidate.
It will then go to an online ballot of the Conservative party membership, with the result to be announced on Friday.
But if the party's MPs get behind just one candidate, we could have a new prime minister by Monday afternoon.
Polling suggests Mr Johnson - who was kicked out of office by his own MPs three months ago - would be favourite to win a members' vote.
He has 46 declared backers so far, according to the running total being kept by the BBC, although other sources suggest it could be higher.
Mt Johnson is heading back to the UK after a Caribbean holiday, with his supporters hoping he will announce a campaign to return to Downing Street.
A close ally, Sir James Duddridge, said he had received a message from the former PM saying: "I'm flying back, Dudders.
We are going to do this.
I'm up for it." His supporters say he is the only contender to have the backing of the voting public after winning the 2019 general election.
Mr Johnson still has a Parliamentary investigation hanging over him over whether he lied to MPs about Covid lockdown-busting parties in Downing Street.
According to the BBC's running total of MPs who have gone on the record with support, Mr Sunak currently has 102 backers.
He has been backed by a number of senior colleagues, including former chancellor and health secretary Sajid Javid, Security Minister Tom Tugendhat and former health secretary Matt Hancock. "It is abundantly clear that Rishi Sunak has what it takes to match the challenges we face - he is the right person to lead our party and take the country forward," Mr Javid said.
House of Commons Leader Penny Mordaunt, who came third in the last leadership election, has 21 backers according to BBC research.
More than half of the Parliamentary party has yet to say which way they will vote.
Launching her campaign on Twitter on Friday, Ms Mordaunt said she would "unite our country, deliver our pledges and win the next [general election]".
Mordaunt backer Conservative MP Bob Seely said "I think we owe the country a collective responsibility to apologise" and said he believes Ms Mordaunt has the best chance of providing "unity and leadership" within the party.
Among those to have ruled themselves out of the race are Defence Secretary Ben Wallace - who has said he is "leaning" towards supporting Mr Johnson - and current Chancellor Jeremy Hunt.
Boris Johnson was ejected from office in July after a string of scandals, but the replacement chosen by the Tory Party, Liz Truss, lasted just 45 days, making her the shortest-serving prime minister in British history.
She stood down on Thursday, after a string of humiliating U-turns forced on her by an adverse reaction to her tax policies in the financial markets.
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