Rishi Sunak is believed to have become the first Tory leadership candidate to have amassed sufficient support to make it to the ballot to run for the leadership of the Conservative Party.
The former chancellor's supporters said he had received nominations from 100 Tory MPs to pass the threshold required way ahead of Monday's deadline.
"Honoured to be the 100th Tory MP to support #Ready4Rishi," senior backbencher Tobias Ellwood tweeted, as other backers of Mr Sunak also said he had crossed the barrier.
Boris Johnson was lagging behind, though like Mr Sunak has not formally declared - unlike Commons Leader Penny Mordaunt who became the first to confirm her candidacy.
But Mr Johnson has told allies he is "up for it" and would fly back from his holiday in the Dominican Republic to enter the race
Mr Sunak will automatically become party leader and prime minister if his opponents - Penny Mordaunt and possibly Boris Johnson - fail in their bids to win 100 nominations from their fellow Conservative MPs.
Mr Johnson and former Mr Sunak have become leading potential contenders to replace Ms Truss as the leadership contest gathers pace.
Announcing her bid for the Tory leadership, Ms Mordaunt tweeted that she has been "encouraged by support from colleagues who want a fresh start, a united party and leadership in the national interest".
"I'm running to be the leader of the Conservative Party and your Prime Minister - to unite our country, deliver our pledges and win the next GE."
The announcement came after she held talks with Jeremy Hunt, in which she assured him he could stay on as Chancellor if she won, with sources saying that she made it clear she would proceed with his budget plan to get the public finances back on track.
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After Ms Truss quit yesterday, ending her six weeks in power, those who want to replace her were trying to find the 100 votes from Conservative MPs needed to run in a contest which the party hopes will reset its ailing fortunes.
With the Conservatives all but facing a wipe out in the next national election according to opinion polls, the race is on to become the fifth British premier in six years.
The winner will be announced on either Monday or Friday next week.
Backing Ms Mordaunt, Tory MP Robbie Moore said that the Conservative Party needs a candidate to unite the party and that he was "incredibly optimistic" that she would get 100 votes.
"Most importantly that (the candidate) is able to deliver on the 2019 manifesto commitments that we made, and that is exactly why I think Penny is the right person to take not just the Conservative Party forward but our nation forward.
"Penny will absolutely get the numbers and I am really excited actually that we have a potential opportunity of having a candidate that is able to unite the Conservative Party, that is able to ensure that we create that stability, that we will also get on with delivering on what our constituents want."
Three senior ministers have pledged support for former prime minister Boris Johnson, such as Defence minister Ben Wallace who ruled himself out of the contest, but told reporters he was leaning towards backing Mr Johnson.
"I feel that I can add the best value in keeping people safe at defence. It's the job that I have been doing and it's the job I intend to stay doing so I'm not going to be standing for prime minister this time."
Speaking to Sky News, he said that Mr Johnson still had "some questions to answer" over the multiple scandals, which resulted in a yet-to-launch investigation in the House of Commons.
Mr Johnson, who left office comparing himself to a Roman dictator brought into power twice to fend off crises, might face difficulty in reaching the 100 votes after his three-year tenure was blighted by scandals and allegations of misconduct.
Secretary of State for Business, Energy and Industrial Strategy Jacob Rees-Mogg this morning announced his support for Mr Johnson on Twitter.
— Jacob Rees-Mogg (@Jacob_Rees_Mogg) October 21, 2022
Asked to comment on speculation that Boris Johnson would launch a bid to return to Downing Street, Taoiseach Micheál Martin said it would be inappropriate for him to comment on the Conservative leadership contest.
"Ireland and Britain are very, very close neighbours," Mr Martin said.
"We enjoy good relationships across the board, economic, social, sporting, culture, and it's extremely important that in the aftermath of a new prime minister being elected, that we get down to work very, very quickly, in respect to resolving issues pertaining to the [Northern Ireland] Protocol, and the relationship between the European Union the United Kingdom," he added.
He said the next prime minister would have to stabilise and facilitate progress in Northern Ireland and to help restore the Executive and Assembly as soon as possible.
UK Secretary of State for Levelling Up, Housing and Communities Simon Clarke also declared his backing for Boris Johnson.
On Twitter this afternoon he said "the comeback will be greater than the setback".
Will Walden, who also worked for Mr Johnson, told Sky News the former prime minister was returning from holiday and was taking soundings, but Mr Johnson himself has not publicly declared whether he is putting himself forward.
Other Tories were aghast at the prospect of his comeback.
Roger Gale warned that Mr Johnson could face a wave of resignations from MPs refusing to serve under him as leader.
While Crispin Blunt told the BBC that Mr Johnson was a "fantastic communicator" but Rishi Sunak was "a much more serious personality" who could impart a "serious message" to the country.
Nigel Mills said it was a "mistake" for him not to back Mr Sunak during the summer's leadership contest, declaring his support.
"He is clearly the Prime Minister we need to restore stability and tackle the many serious challenges facing the country." he said.
A former adviser to Mr Johnson who requested not to be identified, said he was unlikely to reach the target, having alienated dozens of Conservatives during his scandal-ridden tenure.
In what would be an extraordinary comeback, Mr Johnson, who was ousted by MPs just over three months ago, was running high up the ranks alongside Mr Sunak to be crowned the next prime minister.
"I think he's got that proven track record to turn around things. He can turn it around again. And I'm sure my colleagues hear that message loud and clear," Tory MP Paul Bristow said of Mr Johnson on LBC radio.
"Boris Johnson is the character the Labour Party fears, Boris Johnson can win the next general election," he said.
Meanwhile Mr Sunak, the former Goldman Sachs analyst who became finance minister just as the Covid-19 pandemic arrived in Europe, is favourite with bookmakers, followed by Mr Johnson.
Former ministers Sajid Javid and Johnny Mercer have both backed Mr Sunak in the Tory leadership race.
Mr Javid tweeted: "It is abundantly clear that Rishi Sunak has what it takes to match the challenges we face - he is the right person to lead our party and take the country forward.
"The only way for our party to reclaim our values and recover our reputation for stewardship is to move on from the mistakes of the past and come together to focus on the future".
Mr Mercer said he could not put himself or his constituents through another Boris Johnson leadership.
He told BBC Radio 4's PM programme: "Now we need to clearly come behind someone who is capable, professional, calm, competent and will deliver government in a way people expect. For me that is Rishi Sunak".
The contest began yesterday and is widely expected to be a three-horse race between Mr Sunak, Mr Johnson and Ms Mordaunt.
Other candidates could include a representative of the party's right such as Suella Braverman, whose resignation as interior minister helped trigger Truss's downfall.
Ms Braverman told the BBC she would be making a statement later today.
Mr Sunak, proven right in his warnings that Ms Truss's fiscal plan threatened the economy, is the favourite but remains deeply unpopular with some Conservatives after he helped to trigger the summer rebellion against Mr Johnson.
Ms Mordaunt is seen as a fresh pair of hands largely untainted by earlier administrations. But she is also untested and, so far, she is lagging Mr Sunak and Mr Johnson in getting backers.
Whoever takes over the party, they have a mountain to climb to try to restore or renew the reputation of the Conservative Party, which holds a big majority in parliament and need not call a nationwide election for another two years.
Ms Truss quit yesterday after the shortest, most chaotic tenure of any British prime minister after her economic programme shattered the country's reputation for financial stability and left many people poorer.
Ms Truss said she could no longer carry out her programme after her economic plan roiled markets and ended up on the cutting room floor when she was forced to bring in a new finance minister.
"I have therefore spoken to His Majesty the King to notify him that I am resigning as leader of the Conservative Party," said Ms Truss, who was supported only by her husband with her aides and loyal ministers noticeably absent.
The sight of yet another unpopular prime minister making a resignation speech in Downing Street - and the start of a new leadership race - underscores just how volatile British politics has become since the 2016 vote to leave the European Union.
Some Conservative MPs hope the race to replace her will be quick and simple, urging the hopefuls to coalesce around one candidate to reduce the pain of another bruising contest.
Read more: Truss' shortest of political honeymoons
A poll by YouGov has found that three in five voters want an early general election, in line with the angry clamour coming from opposition parties as Britain struggles with a worsening cost-of-living crisis.
Leader of the main opposition Labour party, Keir Starmer, said Britain "cannot have another experiment" after Truss's disastrous tenure.
"This is not just a soap opera at the top of the Tory party -- it's doing huge damage to the reputation of our country," he said.
The party is streaming ahead of the Conservative Party in the latest polls.
"Whether or not a change of leader is going to be sufficient to make the Conservatives actually electorally credible is certainly highly debatable," political scientist John Curtice told LBC.
"The problem for the Conservatives is that the brand of them as a party that can mind the economy... has now been very, very badly tarnished, and it may be very difficult to recover within the space of two years."
Additional reporting Tony Connelly and PA
