London CNN  — 

Britain will have a new prime minister within a week, outgoing leader Liz Truss said in her resignation speech outside 10 Downing Street on Thursday.

The fast-track process is in stark contrast to the contest that catapulted Truss into the hot seat – that lasted six weeks at the height of the summer.

This time, with the Conservatives at rock-bottom in the opinion polls and the markets jittery after a weeks of drama over Truss’ failed economic policy agenda, the party wants a new leader in place as soon as possible and with as little drama as possible.

Graham Brady, the Conservative official responsible for the process, announced the candidates to replace Truss will need get least 100 nominations from the party’s MPs by 2pm local time Monday.

If only one candidate meets that threshold, they will automatically become leader. Otherwise, an online ballot of party members will close on Friday October 28.

The winner of the contest will be the fifth Conservative prime minister in just over six years – and the third within this parliamentary term. But who might the next leader be? Here are some of the main runners and riders:

Rishi Sunak

Though he has yet to formally declare his candidacy, Sunak has already reached the 100-nomination threshold, Britain’s PA Media news agency reported on Friday.

The former Chancellor of the Exchequer (finance minister) has proved to be something of a prophet of the government’s demise, as many of the predictions he made during this summer’s leadership about Truss’s economic plan came to pass.

Conservative leadership candidate Rishi Sunak

Sunak warned that Truss’s unfunded tax cuts would lead to a run on sterling, a panic in the bond market and concern from the International Monetary Fund. Perhaps even he would have been surprised by the pace with which he was proved right.

Sunak has experience of economic crisis-fighting, having guided the UK through the Covid-19 pandemic.

He also secured the most votes from MPs in the last leadership election – comfortably clearing the new threshold with 137 endorsements. Although Truss eventually won the decisive members’ vote, Sunak only lost narrowly – with 43% of the vote.

The trust he has among MPs – and the vindication his predictions have gained – may make him the most likely next set of hands to steer the ship.

Penny Mordaunt

The Leader of the House of Commons may have had a dress rehearsal for being prime minister this week, after stepping in for an absent Liz Truss at a debate.

“The prime minister is not under a desk,” Mordaunt confirmed Tuesday – in a performance that seemed as much about pitching herself as it did about helping the PM.

Mordaunt confirmed in a tweet Friday afternoon that she was running to replace Truss – the first MP to do so.

She promised a “fresh start” for the country, aiming “to unite our country, deliver our pledges and win the next general election.”

Penny Mordaunt

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 newt threshold. She is expected 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.

Like Sunak, she is from the more moderate wing of the party. There was even talk among MPs of the two forming a “dream team” ticket, although this is yet to materialize – and it is unclear if either would accept being chancellor over taking the top job.

Kemi Badenoch

Badenoch came fourth in this summer’s leadership election – securing only 59 votes from MPs – but was consistently rated by pollsters as a favorite among Conservative grassroots members.

One of the younger MPs in the running, Badenoch quickly won the endorsement of long-serving Tory grandee Michael Gove, who praised her as the “outstanding talent” in the party.

Badenoch is from the right of the Tory party – and in her previous leadership bid suggested that the government’s climate targets might prove too costly.

With Truss’s votes from MPs now up for grabs, Badenoch may have an outside chance of clearing the threshold and making it to the members’ vote.

Boris Johnson

Boris Johnson is flying back to the UK from his Caribbean holiday and plans to join the race to replace Truss, Britain’s PA Media news agency reported Friday.

UK Trade Minister Sir James Duddridge, a close Johnson ally, told PA Media that he had been in contact with Johnson via Whatsapp, saying that Johnson had sent him: “‘I’m flying back, Dudders. We are going to do this. I’m up for it’.”

The minister also said via Twitter “I hope you enjoyed your holiday boss. Time to come back. Few issues at the office that need addressing. #bringbackboris.”

Sky News said one of its reporters, who boarded the flight Johnson took, snapped a picture of the former prime minister and his wife Carrie Johnson on board, adding that the couple received several boos from fellow passengers.

Multiple allies have made the case that Johnson could be a unity candidate who could bring stability to the country, despite the fact he resigned in disgrace only a few months ago after a series of scandals came together, making his position untenable.

When asked by CNN how they could justify Johnson standing to be PM again, one MP who campaigned for Johnson in the 2019 leadership campaign, said: “Socialists will destroy our economy and if you don’t understand that then I genuinely fear for our future.”

Another MP who supported Johnson in 2019 said he was the only candidate who could comfortably win over both Conservative MPs and members of the Conservative Party.

Johnson’s closest allies said they were aware he was being actively lobbied in the hours after Truss’ resignation speech, making the case to him that he represented the party’s best shot at stability in the medium term.

In his final speech as prime minister outside 10 Downing Street, Johnson made one of his characteristic allusions to ancient history. He said he would “return to his plough” like the Roman statesman Cincinnatus – suggesting a quieter life on the backbenches. But that’s not how Cincinnatus saw out his days. He was called back from his plough to return to Rome for a second term – this time as a dictator.

Some suspect that the new 100-vote threshold is an attempt by the Conservative Party to render another Johnson term impossible. But Johnson’s campaign has already started to gather momentum. A number of prominent Conservative MPs announced Friday that they will back him – even though Johnson has yet to confirm he will stand.

As Conservative MPs are currently facing electoral oblivion, their desire for self-preservation should not be underestimated. Less than three years ago Johnson delivered an 80-seat majority, and the right of the Tory party may think he is the only candidate capable of saving their jobs.

If Johnson was to secure the required 100 votes from MPs, he would be expected to perform extremely well in a vote by the party membership.

Grant Shapps

It is a sign of the disorder of the last days of Truss’s government that she elevated Grant Shapps to home secretary – despite not offering him a ministerial role of any sort when she first took office.

Grant Shapps

Shapps served as transport secretary under Boris Johnson. He put himself forward to succeed him in the previous leadership election – only to withdraw from the race three days later, after failing to secure the requisite 20 MPs’ votes to proceed to the next round.

The new threshold will likely prove too high for Shapps – but his criticism of Truss’ government from the outset may have won him the support of more MPs than last time.

Other names in the hat

Suella Braverman’s resignation as home secretary on Wednesday night may have been a precursor to a possible leadership bid. The former attorney-general has not run before – but with her hard-line stance on immigration, might look set to drag the party further to the right.

Tom Tugendhat emerged as a surprise favorite among Tory members and the wider public, despite only coming fifth in the last leadership election. Having not served as a cabinet member before that contest, Tugendhat distanced himself from the moral mess of Johnson’s government and promised a “clean start” for Britain. After serving in Iraq and Afghanistan, Tugendhat was made security minister by Truss.

Ben Wallace, defense secretary and another ex-military man, was tipped to succeed Johnson in the last leadership contest – polling extremely well among Conservative members. However, he never ran in that election, and he has now ruled himself out of this race.

Former prime minister Theresa May has also been floated as a possible “unity” candidate to succeed Truss. May tried to bring together the warring wings of the Conservative party over Brexit, in move that ultimately saw her replaced by Boris Johnson. As the party has proven unable to resolve its disputes this time round, another attempt at compromise may soon be in order.

CNN’s Luke McGee contributed to this report.