Rishi Sunak supporters claimed on Saturday that the Indian-origin former chancellor has hit the threshold of 100 members of Parliament required to make it to the shortlist for the race to replace Liz Truss as the Conservative Party leader and British Prime Minister.

The 42-year-old frontrunner's tally continues to grow as former prime minister Boris Johnson flew back from his Caribbean holiday and his camp also claimed that he has the requisite minimum of 100 MPs for a possible leadership bid.

Neither Sunak nor Johnson has officially declared their intention to contest the leadership election, with Leader of the Commons Penny Mordaunt the only candidate to formally announce her candidacy so far.

However, the former finance minister has taken a comfortable lead with some heavyweight Tory ministers and MPs from different factions of the Tory party throwing their weight behind him and the betting odds also continuing to rise in favour.

"Rishi had the right plan in the summer and I think it is the right plan now," former deputy prime minister Dominic Raab told the BBC.

"I think he is the best placed candidate to provide some stability, to provide confidence for the millions of workers and businesses up and down the country," he said.

Raab warned against a comeback by the partygate scandal-hit former leader Johnson, just over three months after being forced to resign, pointing out that the UK Parliament is still conducting an inquiry into whether he misled Commons over the lockdown law-breaking parties at Downing Street.

"We cannot go backwards. We cannot have another episode of the Groundhog day, of the soap opera, of partygate. We must get the country and the government moving forward," he said.

It came as Johnson was pictured by 'Sky News' on a flight back to London from the Dominican Republic with his wife and children, having indicated to allies of his intention to have another go at 10 Downing Street.

The tally of MPs publicly supporting him for the comeback publicly stands at 53, with former Home Secretary Priti Patel among his high-profile backers and his loyalists are confident that he would hit the 100-MP mark required by 2 pm local time on Monday to make the shortlist.

"Boris has the mandate to deliver our elected manifesto and a proven track record getting the big decisions right. I'm backing him in the leadership contest," she tweeted.

If only one candidate is left standing by then on Monday, then the online Tory membership vote due to happen next week for a winner announced next Friday, will not have to take place.

If any one candidate secures 156 MPs, then the race would be automatically narrowing to a shortlist of two given the Tory MPs Commons tally of 357 and not enough to support a third candidate's minimum 100.

Meanwhile, there is widespread speculation among backers of both candidates that Sunak and Johnson may strike some kind of a deal.

According to The Daily Telegraph, Sunak is expected to reject any offer of a Cabinet role during the leadership race, given he is the frontrunner and had resigned from a Johnson government once already this year.

But no categorical statement of rejection has been issued, leaving open a job acceptance if Johnson does indeed emerge triumphant.

Sunak is being urged to offer Johnson a Cabinet role, such as home secretary, to agree to get him to stand back ahead of any voting with Tory members.

Sunak had stormed ahead in the initial round of voting among Tory MPs and lost to Truss in the leadership contest last month in the vote by the 170,000 Tory membership, who either favoured Truss' now-failed tax-cutting pledges or viewed Sunak as having hastened Johnson's downfall by resigning as chancellor.