Boris Johnson dropped out of the race, and Rishi Sunak has secured at least 165 supporters

Former prime minister Boris Johnson has ended his bid to return to power within months of being ousted, claiming that although he had the numbers, he would not run to replace Liz Truss, who resigned as Conservative party leader on Thursday after just 45 days in office. Johnson claimed to have won the support of 102 colleagues – two clear of the threshold needed – but only about 60 had publicly stated their support for him.

Johnson said he reached the decision reluctantly after recognising he would not lead “a united party in parliament”. He said, “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,” and, “You can’t govern effectively unless you have a united party in parliament.”

Former chancellor Rishi Sunak praised Johnson on Twitter late on Sunday, noting Johnson “delivered Brexit and the great vaccine roll-out” and “led our country through some of the toughest challenges we have ever faced”. Britain would “always be grateful to him for that”, he wrote, adding, “I truly hope he continues to contribute to public life at home and abroad.”

Sunak appears set to become party leader and prime minister. He ended Sunday with more than 165 supporters ahead of Monday’s nomination deadline. Sunak came second in the race against Truss over the summer.

The chances of a general election have risen, according to some estimations. Johnson supporter Nadine Dorries has said an election was now “impossible to avoid”. Labour, which has opened up huge poll leads, is demanding an election. Angela Rayner, the deputy leader, said: “The Tories are about to hand the keys of the country to Rishi Sunak without him saying a single word about how he would govern. No one voted for this. Perhaps it’s not surprising he’s avoiding scrutiny: after all, he was so bad that just a few weeks ago, he was trounced by Liz Truss.”

Contender Penny Mordaunt, who missed out on the last contest’s run-off by just eight votes, will now come under pressure to concede rather than force the contest to a vote of members. However a source on the Mordaunt campaign insisted her campaign was continuing and that she wanted to get on the ballot so party members could decide the result.

Johnson and Sunak held talks late into Saturday night, according to reports. The ex-leader also reportedly spoke on Sunday to Mordaunt, who was said to have rebuffed his calls to back him, noting her supporters were likely to split more for Sunak.

Mordaunt could yet win over any former Johnson supporters who want to stop Sunak. Each needs to submit nominations by 2pm on Monday. If both get more than 100 nominations, 150,000 Tory members will be asked to decide the result.

Sunak launched his official campaign with a declaration that “fixing the economy” was his priority, but he gave no media interviews or formal manifesto. If Sunak succeeds on Monday he will become Britain’s first non-white PM, and as a Hindu, his victory will be sealed on Diwali.

Mordaunt spoke to the BBC’s Laura Kuenssberg on Sunday, telling her she would be a “halfway house” between Sunak and Johnson but refused to be drawn on any economic policies or decisions on tax and spending.

A survey by Conservative pollster James Johnson still found all three Tory candidates had negative favourability. The survey found that Johnson was on -24, Mordaunt -15 and Sunak -2.