Penny Mordaunt has become the first candidate to formally announce her bid to replace Liz Truss as prime minister.
The Commons leader, who came third in the last contest, said she would offer a “fresh start” and a “united party”.
Rishi Sunak is also expected to stand but has yet to officially declare his bid. Boris Johnson is also believed to be considering a dramatic return to No.10.
Candidates need to secure the support of 100 Tory MPs to make it onto the ballot, with the winner due to be announced by next Friday.
“I’ve been encouraged by support from colleagues who want a fresh start, a united party and leadership in the national interest,” she said.
“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 (general election).”
HuffPost UK understands Mordaunt met with Jeremy Hunt on Friday and promised he would remain as chancellor should she win the leadership. A source said her “first priority” was the “economy and Treasury”.
Hunt is due to deliver the long-awaited medium-term fiscal plan on October 31.
It comes as allies of Johnson came out in public to back a bid by the former PM to return.
Cabinet ministers Jacob Rees-Mogg, Ben Wallace and Simon Clarke all said they wanted Johnson back.
But Foreign Office minister Jesse Norman argued that selecting the former PM would be “absolutely catastrophic”.
And veteran backbencher Sir Roger Gale, a long-time critic of Johnson, warned that if he succeeds he could be met with a wave of resignations by Tory MPs.