Britain's former Finance Minister Rishi Sunak has confirmed he is standing to replace Liz Truss as prime minister.

"The United Kingdom is a great country but we face a profound economic crisis," he said on Twitter.

"That's why I am standing to be leader of the Conservative Party and your next prime minister."

Sunak was defeated by Truss in the race to replace Boris Johnson in September after losing an election held by members of the Conservative party across the country.

Even before declaring on Sunday, Sunak had received more support from Conservative lawmakers than his rivals - former defense minister Penny Mordaunt and ex prime minister Boris Johnson, who is attempting a comeback.

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Sunak quit Johnson's government in July, helping trigger a rebellion that bought him down.

"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," he said in a statement.

"I am asking you for the opportunity to help fix our problems."

Meanwhile, Boris Johnson was fighting on Sunday to get enough support to make a shock return as Britain's prime minister after prominent figures on the right-wing of the Conservative Party coalesced around the man once accused of betraying him, Rishi Sunak.

Not the time for 'Boris's style'

Johnson was prime minister from 2019 to 2022 until he was forced to resign over a string of scandals. One-time backers are now urging him to stay out of the race to replace his successor Liz Truss, who only lasted six weeks in office.

Johnson is still facing an investigation into whether he misled parliament when he was last in power, and several former backers have said that would guarantee a return to the constant state of drama that accompanied his previous premiership.

"This isn't the time for Boris's style," Steve Baker, an influential lawmaker on the right of the party, told UK's Sky News. "I'm afraid the trouble is because of the privileges vote, Boris would be a guaranteed disaster."

The prospect of another Johnson premiership is a polarizing issue for many in the Conservative Party, which is deeply divided after seeing off four prime ministers in six years.

For some Conservative lawmakers, Johnson is a vote-winner, able to appeal across the country with his brand of energetic optimism. For others, he is a toxic figure who would struggle to unite the party and so might undermine efforts to build a stable leadership to calm rattled financial markets.

Chris Heaton-Harris, the minister for Northern Ireland, said Johnson did have enough supporters to enter the ballot on Monday. "We do have the numbers," he told Sky News. "That's not an issue."

Boris 2.0

Johnson was given a boost on Sunday when Nadhim Zahawi, who was briefly finance minister earlier this year, backed him to return to Downing Street.

"When I was Chancellor, I saw a preview of what Boris 2.0 would look like," he said on Twitter. "He was contrite and honest about his mistakes. He'd learned from those mistakes how he could run Number 10 and the country better.

"With a unified team behind him, he is the one to lead us to victory and prosperity."

Under the rules, if only one candidate passes the threshold of 100 lawmakers on Monday then they will be declared as prime minister. If more than one clears the hurdle, they will face a vote of around 170,000 signed-up members of the Conservative Party, with the winner announced on Friday.