Rishi Sunak has formally announced that he is standing for Tory leader and prime minister.
The former chancellor said he wanted to “fix the economy, unite our party and deliver for our country”.
Sunak said there would be “integrity, professionalism and accountability at every level of the government” in an apparent attempt to distinguish himself from his predecessors.
In a tweet posted on social media, Sunak wrote: “I served as your chancellor, helping to steer our economy through the toughest of times.
“The challenges we face now are even greater. But the opportunities — if we make the right choice — are phenomenal.
“I have the track record of delivery, a clear plan to fix the biggest problems we face and I will deliver on the promise of the 2019 manifesto.”
This weekend there had been reports that Sunak had held talks Boris Johnson in an attempt to “unify” the party.
Sunak’s decision to declare his candidacy suggests the talks had broken up without an agreement.
To get on the leadership ballot, candidates must secure the nominations of 100 MPs.
The former chancellor is currently ahead with 133 public declarations, with Johnson on 55 and Penny Mordaunt on 23.
In a major boost for his campaign, Sunak, who lost out to Liz Truss in the last leadership race, secured support from leading figures on the right of the party.
Brexiteer rebel Steve Baker and international trade secretary Kemi Badenoch both pledged their support for Sunak in the interest of party unity.
Shortly after Sunak declared, home secretary Grant Shapps confirmed that he was also backing the former chancellor.
“We need someone who can provide stability and proven economic competence in these challenging times, and Rishi Sunak is that person,” he tweeted.
A potential comeback for Johnson, who was ousted by his own MPs just three months ago, has exposed deep splits in the Conservative party.
Baker warned that the controversy surrounding Johnson over the partygate scandal and the privileges committee investigation into whether he misled MPs on the matter meant “Boris would be a guaranteed disaster”.
Damian Green, Theresa May’s deputy prime minister, said he did not think another Johnson premiership “would work”.
He told Sophy Ridge On Sunday: “We saw what happened last time. The government literally fell apart with 60-odd resignations so I just do not think it would work. I think particularly not at this time.
“We all know there’s this standards committee investigation going on and as long as that is going on there is the possibility if he became leader again, then in a couple of months time we’d all be here again and, absolutely, we should not put the country through that.”
Earlier on Sunday Johnson’s allies insisted he would stand in the Tory leadership race and that he had the numbers to get on to the ballot.
Business secretary Jacob Rees-Mogg told the BBC’s Laura Kuenssberg: “I have been speaking to Boris Johnson, and clearly he’s going to stand, there’s a great deal of support for him”.
Northern Ireland secretary and former chief whip Chris Heaton-Harris, who is also backing Johnson, said he thought Johnson would run, telling Sky News: “This is a time when we need a big player like Boris in our politics so I think he will.”
“We definitely have enough numbers – we’ve already booked the appointment with Bob Blackman, the secretary of the 1922 Committee, tomorrow.”