Commons Leader Penny Mordaunt said she is running to replace Liz Truss as Tory leader and prime minister.
Announcing her bid to be the next PM, she pledged to “unite our country, deliver our pledges and win the next GE”.
In a tweet, she vowed to give the party a “fresh start” and provide “leadership in the national interest”.
Ms Mordaunt has already held talks with Jeremy Hunt and assured him he can stay on as Chancellor if she becomes prime minister, it is understood. She has also assured him that she would deliver the medium-term fiscal plan as proposed on October 31, a source close to the Commons leader said.
Former cabinet minister Andrea Leadsom backed Ms Mordaunt to be the new Tory leader, citing her "compassion" and "determination".
Quoting a post from Ms Mordaunt announcing her bid, Ms Leadsom tweeted: "Delighted by this!
"Penny has the experience, the compassion and the determination to lead our country to a bright future!"
Ms Mordaunt, 49, finished third in the last leadership election.
It comes after three Cabinet ministers threw their weight behind Boris Johnson - who is yet to declare - on Friday afternoon.
Business Secretary Jacob Rees-Mogg, Defence Secretary Ben Wallace and Levelling Up Secretary Simon Clarke all announced they would back Mr Johnson were he to run for No10 once again. He resigned just under four months ago after an unprecedented slew of ministerial resignations from his Government, prompted by a string of scandals and allegations of sleaze.
There has also been no declaration yet from former Chancellor Rishi Sunak, who did not answer questions from reporters as he left his home on Friday morning.
Veteran backbencher Sir Roger Gale, a long-time critic of Mr Johnson, has warned that if the former PM succeeds he could be met with a wave of resignations by Tory MPs.
Former environment secretary George Eustice said “you can’t turn the clock back” as he confirmed he would not be supporting a return of Mr Johnson.
“The reason Boris Johnson resigned in the end is that half of his own government ministers had resigned from his government, saying they had no confidence in him and probably around two-thirds of the Conservative parliamentary party were poised to vote against him in a vote of no confidence so he had no option but to resign,” he said.
“I don’t see how it’s possible for him to find a route back just two months later, to be that unifying figure when the vast majority of Conservative MPs were ready to vote to say they had no confidence in him as recently as July.”
