Boris Johnson on Sunday night pulled out of the race to become Britain’s next prime minister, after rivals claimed he would fail to reach the necessary threshold of 100 nominations from Tory MPs.

After a day of frantic attempts by Johnson to bolster his support, he announced at 9pm that he was abandoning his bid to make a comeback as prime minister just weeks after he quit Number 10.

His withdrawal from the contest leaves Rishi Sunak, former chancellor, as the clear frontrunner to succeed Liz Truss as Conservative leader and prime minister.

His only declared rival for the job, Leader of the House of Commons Penny Mordaunt, has failed to so far win the backing of more than 30 Tory MPs.

If she does not reach the threshold of 100 nominations by 2pm on Monday, Sunak will become prime minister.

Johnson insisted in a statement that he would have reached the 100 nominations needed, but had withdrawn from the contest in the interests of party unity.

“In the last few days I have been overwhelmed by the number of people who suggested that I should once again contest the Conservative party leadership, both among the public and among friends and colleagues in parliament,” he said.

“I have been attracted because I led our party into a massive election victory less than three years ago — and I believe I am therefore uniquely placed to avert a general election now. A general election would be a further disastrous distraction just when the government must focus on the economic pressures faced by families across the country.

“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.”

Tory MPs in rival camps believe that Johnson’s claim to have over 100 backers might have been disproved when nominations closed on Monday. Johnson claimed he would have won the contest, once it was thrown open to members of the party.

“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,” he said.

“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 and Penny — 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.”