Rishi Sunak could be announced as Britain’s next prime minister, hours after Boris Johnson ruled himself out of the race for No 10 amid uncertainty over rival Penny Mordaunt’s prospects of securing enough support from MPs.

Tory MPs will choose who they want to become their new leader in the first stage of the contest on Monday, when both remaining candidates receive the 100 nominations needed to reserve a place on the ballot.

If Ms Mordaunt does not reach this amount by 2pm or withdraws from the election, Mr Sunak will take over the leadership of the party without the need for a vote.

It came after Mr Johnson abruptly withdrew from the race – never formally entered, claiming he had the numbers but admitting he could not unite his embattled party.

Rishi Sunak leaves his election office in London (Belinda Jiao / PA)

In a statement on Sunday night, he said there was a “very strong possibility” he could be back in No.10 by the end of the week if he stood.

However, his attempts to “bond” with his rivals – Mr Sunak and Ms Mordaunt – to work together in the national interest were unsuccessful, so he withdrew.

While Mr Sunak, the former chancellor, already has more than 140 public declarations of support, Ms Mordaunt, the House leader, had fewer than 30 on Sunday night.

Her team hopes Mr Johnson’s departure will see a streak of MPs who have supported him or who have not yet announced their defection.

A campaign source confirmed she was still in the running on Sunday, claiming she was the candidate Labor feared most.

Penny Mordaunt leaves BBC Broadcasting House in London (Belinda Jiao / PA)

“Penny is the unifying candidate most likely to hold the wings of the Conservative Party together and the poll shows she is the most likely candidate to retain the seats won by the Conservative Party in 2019,” the source said.

However, one senior minister who supported Mr Johnson – Chancellor of the Duchy of Lancaster Nadhim Zahavi – said he would now back Mr Sunak.

“Rishi is very talented and will command a majority in the Parliamentary Conservative Party and will have my full support and loyalty,” he tweeted.

Ms. Mardaunt has limited time to obtain the necessary nominations. If unsuccessful, Mr. Sunak will be declared leader without a contest.

If she does get the numbers, MPs will decide which of the two candidates they prefer in an “indicative” vote.

There will then be a final online poll of party members to determine the result, with results to be tallied on Friday – unless one of the candidates withdraws.

There are some in the party who would like to see an uncontested “coronation” to avoid a repeat of what happened to Liz Truss, when party members voted in a leader who did not have the support of MPs.

Ms Mordaunt could come under pressure to withdraw if she trails Mr Sunak significantly in the MP poll – if it goes to a vote, even though she is popular with the Tories.

At the same time, however, many activists – many of whom loathe Mr Sunak for his role in ousting Mr Johnson – will be furious if they are denied the vote.

In a statement on Sunday night, Mr Johnson said he was “surprised” by the support he had received from people calling for him to run just weeks after his own MPs were forced out of office following one scandal.

Had he run, he said there was a “very good chance” his friends would have returned him to Number 10 by the end of the week and that he would be “well placed” to lead the party to victory in the 2024 general election.

However, he said he concluded that “it just wouldn’t be right.”

“You cannot govern effectively if you don’t have a single party in parliament,” he said.

“And although I reached out to Risha and Penny – because I hoped we could come together in the national interest – we unfortunately couldn’t work out a way to do that.

“I believe I have something to offer, but I’m afraid it’s just not the right time.”

Some MPs were skeptical of his claim that he had received the 100 nominations needed to move forward, with the number of public declarations of support far fewer.

Some in Westminster suspected he had decided to quit rather than face the humiliation of admitting he couldn’t get the numbers.

Former minister Tobias Ellwood says ‘the country does not want further economic instability’ (Kirsty O’Connor/Pennsylvania)

Tory MP Tobias Ellwood, who supports Sunac, meanwhile rejected Labor calls for a general election.

“The country does not want further economic instability,” the chairman of the Commons Defense Committee told the BBC.

“And if there were a general election, it would be unclear in which direction the country would move, there would be more upheaval, the markets would be scared again.

“There is likely to be a sharp fall in sterling, which will lead to higher interest rates (and) higher mortgages. This is not the kind of leadership that the country wants.”