Boris Johnson says he will not make another tilt at the British prime ministership, despite claiming he has the numbers to “be back in Downing Street on Friday”.

In a statement, the former PM has ruled himself out of the race to be Britain’s fourth prime minister of 2022, after days in which his backers worked behind the scenes to count the support.

Mr Johnson had raced home from a holiday in the Caribbean to secure the backing of 100 MPs to enter Monday’s contest to replace Liz Truss, the woman who succeeded him in September after he was forced to quit over a string of scandals.

He said he was “overwhelmed” by the number of people who suggested he should contest the Conservative Party leadership and believed he could win. But he had failed to persuade either the No.1 contender for the leadership, Rishi Sunak, or Mr Sunak’s main rival Penny Mordaunt, to come together “in the national interest”.

“I believe I am well placed to deliver a Conservative victory in 2024 – and tonight I can confirm that I have cleared the very high hurdle of 102 nominations, including a proposer and a seconder, and I could put my nomination in tomorrow,” he said in a statement released Monday morning (Australian time).

“There is a very good chance that I would be successful in the election with Conservative Party members – and that I could indeed be back in Downing Street on Friday.

“But in the course of the last days I have sadly come to the conclusion that this would simply not be the right thing to do.

“You can’t govern effectively unless you have a united party in parliament.

“And though I have reached out to both Rishi [Sunak] and Penny [Mordaunt] – because I hoped that we could come together in the national interest – we have sadly not been able to work out a way of doing this.

“Therefore I am afraid the best thing is that I do not allow my nomination to go forward and commit my support to whoever succeeds.

“I believe I have much to offer but I am afraid that this is simply not the right time.”

Mr Johnson’s decision leaves Mr Sunak, a former British Treasury chief, as the frontrunner to replace Ms Truss.

Mr Sunak garnered the public support of more than 100 Tory MPs to forge ahead of his two main rivals – Mr Johnson and Ms Mordaunt.

Mr Sunak, 42, was runner-up after Ms Truss in this summer’s Tory leadership race to replace Mr Johnson after he was forced out by a string of ethics scandals.

Mr Sunak has the backing of at least 124 Conservative MPs, according to unofficial tallies compiled by British news organisations. That’s well ahead of the 100 nominations required to qualify.

According to the party’s rules, if only one candidate secures the backing of 100 Conservative MPs, they will be named prime minister later on Monday.

If two candidates pass the threshold, they will go forward to a vote of the party membership, with the winner announced on Friday, just days before new finance minister Jeremy Hunt lays bare the state of the country’s finances in a budget plan due to be released on October 31.

“There will be integrity, professionalism and accountability at every level of the government I lead and I will work day in and day out to get the job done,” Mr Sunak said in a statement.

Northern Ireland minister Steve Baker, a former backer of Mr Johnson and an influential politician within the Conservative Party, earlier warned a Johnson comeback would be a “guaranteed disaster.”

“This isn’t the time for Boris and his style,” Mr Baker told Sky News on Sunday.

“What we can’t do is have him as prime minister in circumstances where he’s bound to implode, taking down the whole government … and we just can’t do that again.”

But Mr Johnson won the backing of several senior Conservatives, including Nadhim Zahawi, another former Treasury chief.

“He was contrite and honest about his mistakes. He’d learned from those mistakes how he could run No 10 and the country better,” Mr Zahawi said.

Ms Truss quit last Thursday after a turbulent 45 days, conceding that she could not deliver on her botched tax-cutting economic package. she was forced to abandon it after it sparked fury within the Conservatives and weeks of turmoil in financial markets.

Mr Sunak, who was Treasury chief from 2020 until this northern summer, steered Britain’s slumping economy through the coronavirus pandemic. He quit in July in protest at Mr Johnson’s leadership.

In the summer contest to replace Mr Johnson, Mr Sunak called promises by Ms Truss and other rivals to immediately slash taxes reckless “fairy tales” and argued that surging inflation must be controlled first.

Tory voters backed Ms Truss over Mr Sunak. But he was proved right when Ms Truss’ unfunded tax-cutting package triggered chaos in the markets in September.

-with AAP