London CNN  — 

Britain’s former finance minister Rishi Sunak has emerged as the front-runner to be the country’s next prime minister, after Boris Johnson dramatically dropped out of the race to be Conservative Party leader.

The current prime minister, Liz Truss, resigned last week after just six disastrous weeks in office. Graham Brady, the Conservative official responsible for the leadership contest, said a new prime minister would be in place by Friday.

Candidates to replace Truss have until 2 p.m. (9 a.m. ET) on Monday to secure the support of 100 Conservative Party lawmakers to enter the race to become the party – and the country’s – new leader.

Johnson’s decision to withdraw from the contest leaves Sunak, the former Chancellor of the Exchequer, competing against Leader of the House of Commons Penny Mordaunt.

If both candidates secure the backing of 100 MPs, it will then be up to the roughly 172,000 members of the Conservative Party to pick the new leader in online voting.

As of Monday morning, Sunak was the only one of the pair to have met the necessary threshold of 100 nominations.

If Sunak remains the only one to have reached this number of backers, he will automatically become the Conservative Party’s new leader. The party’s parliamentary majority ensures he will also become the country’s next prime minister.

Boris Johnson's withdrawal from the race leaves Rishi Sunak (L) and Penny Mordaunt (R) as the remaining contenders.

After days of speculation, Sunak officially declared he’d be standing in the contest on Sunday, writing on Twitter: “The United Kingdom is a great country but we face a profound economic crisis. That’s why I am standing to be Leader of the Conservative Party and your next Prime Minister. I want to fix our economy, unite our Party and deliver for our country.

“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.”

Many of Johnson’s supporters have blamed Sunak for helping to oust Johnson from power in July in the wake of a series of scandals, but on Sunday evening he paid tribute to the former PM after he pulled out of the race.

“Boris Johnson delivered Brexit and the great vaccine roll-out. He led our country through some of the toughest challenges we have ever faced, and then took on Putin and his barbaric war in Ukraine. We will always be grateful to him for that,” 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.”

In his statement announcing his withdrawal from the race, Johnson claimed to have secured the nominations of 102 MPs, even though the number of lawmakers who had publicly announced their support for him was much lower. And he took a swipe at Sunak and Truss after failing to convince them to support his comeback.

“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,” Johnson 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 (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,” he added. “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.”

Mordaunt was the first to declare her candidacy on Friday. She promised a “fresh start” for the UK, aiming “to unite our country, deliver our pledges and win the next general election.” She is also running under a catchy hashtag: “PM4PM.”

She too praised Johnson for his decision to withdraw from the contest, saying the former PM “put country before party, and party before self.”

“He worked to secure the mandate and the majority we now enjoy. We should put it to good use, and I know he will work with us to do so,” she said on Twitter.

Mordaunt came third in the last leadership election, narrowly missing out on being put before the members. With 105 votes from MPs in the last election, she too is expected to clear the new threshold, and to perform well among the party membership, in part due to her military credentials. Mordaunt is a reservist of the Royal Navy and served a short spell as Secretary of State for Defense.

The last time the Conservatives held a leadership race – following the demise of Johnson’s government – Truss came first, Sunak second and Mordaunt third. Johnson did not run.

Keir Starmer, leader of the main opposition Labour Party, renewed his calls for a general election on Sunday, after claiming people were “fed up to the back teeth” with the Conservative leadership and the consequences of their government’s decisions.

“There is a choice to be made. We need a general election! Let the public decide … Do they want to continue with this utter chaos, or do they want stability under a Labour government?” Starmer asked during a BBC interview.

CNN’s Allegra Goodwin, Jorge Engels and Sharon Braithwaite, Luke McGee, Christian Edwards, Duarte Mendonca, Lindsay Isaac, Jennifer Hauser and Tara Subramaniam contributed to this report.