Boris Johnson has pulled out of the Conservative leadership race.
His withdrawal leaves the path to No 10 open to Rishi Sunak.
The former prime minister had the public backing of 59 Tory MPs but had to reach 100 to proceed in the race.
Mr Sunak, the former chancellor, has more than 140 backers, a significant lead over Penny Mordaunt, the only other remaining candidate. If he wins, Mr Sunak will become the third prime minister this year.
Path to No 10 opens for Sunak – live updates
In a statement, Mr Johnson said he had “cleared the very high hurdle of 102 nominations”, but came to the conclusion that “this is simply not the right time”.
He said he had “reached out” to leadership rivals Mr Sunak and Ms Mordaunt to see if they could work together in the national interest, but it had not proved possible.
He added that although he was “attracted” to run because of the support from his colleagues, “you can’t govern effectively unless you have a united party in parliament”.
In response, Mr Sunak tweeted: “Although he has decided not to run for PM again, I truly hope he continues to contribute to public life at home and abroad”.
One of Mr Johnson’s key allies told Beth Rigby, Sky News’ political editor, that he had said he thinks two-thirds of the party are against him and that he would be unable to govern like Liz Truss.
Another ally said the anti-Johnson coalition in parliament is “very loud” and “very motivated”.
The first Johnson backer to publicly switch to Mr Sunak was Nadhim Zahawi, the former chancellor, who tweeted “a day is a long time in politics”.
He came out for Mr Johnson this morning.
Ms Mordaunt, the leader of the Commons, has the public backing of 24 MPs. She needs 100 by 2pm tomorrow to stay in the race.
On Sunday, several long-time allies of Mr Johnson, including Suella Braverman and Steve Baker, threw their support behind Mr Sunak.
Mr Baker, the former head of backbench Brexiteers, warned that a potential comeback by Mr Johnson would be a “guaranteed disaster”.
Mr Sunak had already met the threshold needed to make it to the next stage of the race before his candidacy announcement this morning.
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Mr Johnson flew back from a holiday in the Caribbean on Saturday and did not formally announce his candidacy in the race.
He secured the public backing from big names in the cabinet, including Foreign Secretary James Cleverly and Mr Zahawi, the chancellor of the Duchy of Lancaster.
Mr Johnson, who was ousted by his own colleagues in July, was widely expected to run to replace Liz Truss who quit on Thursday after her tax-cutting economic package caused turmoil in financial markets and obliterated her authority inside the party.