Rishi Sunak is believed to have become the first Tory leadership candidate to have amassed sufficient support to make it to the ballot of Conservative colleagues.
The former chancellor’s supporters said on Friday night he had received nominations from 100 Tory MPs to pass the threshold required way ahead of Monday’s deadline.
Boris Johnson was lagging behind, as was Commons Leader Penny Mordaunt who became the first to confirm her candidacy to replace Liz Truss as Prime Minister.
But Mr Johnson has told allies he is “up for it” and was flying back overnight from his holiday in the Dominican Republic to enter the race and attempt an extraordinary comeback.
Sky News photographed the ex-PM and his wife Carrie Johnson on an overnight British Airways flight back from the Dominican Republic with their children and said the MP received “one or two boos” as he boarded.
There is speculation among backers of the old Downing Street neighbours that they could strike a deal to stand together, but this would need them both to bury a lot of rancour.
Mr Sunak, who came second against Ms Truss in the last race six weeks ago, has around 82 public declarations, far ahead Mr Johnson’s 48, while Ms Mordaunt struggles on 18.
Rishi Sunak was unsuccessful in his previous attempt to become PM.
But a source in the former chancellor’s camp told the PA news agency he had won the backing of 100 colleagues to make it to the next stage.
Tory MPs will vote on Monday, and two candidates will be put forward to the Tory membership unless one pulls out, with a result being announced on Friday.
Mr Johnson won the support of six Cabinet ministers: Ben Wallace, Jacob Rees-Mogg, Simon Clarke, Chris Heaton-Harris, Alok Sharma and Anne-Marie Trevelyan.
But former Tory leader Lord William Hague warned Mr Johnson’s return would lead to a “death spiral” for the Conservative Party.
“And I think it’s possibly the worst idea I’ve heard of in the 46 years I’ve been a member of the Conservative Party,” he told Times Radio.
Neither Mr Johnson or Mr Sunak have officially launched their campaigns, unlike Ms Mordaunt who put her name forward with a pledge to re-unite the bitterly divided Tory Party.
The Leader of the House – who finished third in the last leadership election – said she had been encouraged by the support she had received from fellow Conservative MPs.
“I’ve been encouraged by support from colleagues who want a fresh start, a united party and leadership in the national interest,” she said in a statement posted on social media.
“I’m running to be the leader of the Conservative Party and your Prime Minister – to unite our country, deliver our pledges and win the next GE (general election).”
The announcement from Ms Mordaunt came after she assured Jeremy Hunt he could stay on as Chancellor if she won.
Mr Johnson’s supporters have been urging him to mount a comeback six weeks after he left No 10 after being forced to resign by a Cabinet rebellion over a series of scandals.
Boris Johnson is rumoured to be making a charge for the Tory leadership.
Ally Sir James Duddridge said Mr Johnson has told him he is “up for it”, and would head back from his Caribbean holiday and land in the UK on Saturday.
The trade minister told the PA news agency: “I’ve been in contact with the boss via WhatsApp.
“He’s going to fly back. He said, ‘I’m flying back, Dudders. We are going to do this. I’m up for it’.”
Some MPs have warned they would resign the Tory whip and sit in the Commons as independents if Mr Johnson returns to Downing Street.
Defence Secretary Ben Wallace ruled himself out as a candidate, saying he was “leaning towards” backing his long-time ally Mr Johnson.
Mr Wallace praised his record on investing in defence and pointing to the “huge majority” he won in 2019.
He told broadcasters: “This will be potentially our third prime minister since the general election of 2019 – that means we have to think about that legitimacy question that the public will be asking themselves, and also about who could win the next election – that’s obviously important for any political party at the time.”
Candidates have until 2pm on Monday to secure the 100 nominations, limiting the ballot to a maximum of three candidates.