Penny Mordaunt is the first Tory leadership contender to confirm she is running for No 10, promising she can deliver a “fresh start” for the party.
The Commons leader – who finished third in the last race, in July – is likely to face Rishi Sunak and possibly Boris Johnson, but neither has made an announcement yet.
Ms Mordaunt is believed to have told Jeremy Hunt he will remain as chancellor – and that there will be no delay to his de facto Budget planned for 31 October.
She tweeted: “I’ve 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 [general election].”
Ms Mordaunt has ground to make up, having the declared backing of only 18 fellow Conservatives and needing 100 to clear the bar for nominations on Monday.
This year’s first leadership campaign saw her star rise dramatically at the start of the contest – only for her to be pegged back and overtaken by Liz Truss.
A fresh opinion poll, for Opinium, has found the public prefers her to Mr Johnson, but only by the narrow margin of 36 per cent to 33 per cent.
She lacks popularity in a head to head with Mr Sunak, who is preferred by 45 per cent of people against only 23 per cent for Ms Mordaunt, according to the survey.
The navy reservist wowed Tory MPs with a polished performance defending Ms Truss in the chamber last week, but may be vulnerable to the charge of lacking experience.
The only department she has run is defence – and only very briefly – at a time when her colleagues are likely to be wary of taking a gamble, after the disastrous reign of the current, departing prime minister.
Andrea Leadsom, a leadership contender in 2017, tweeted that she was “delighted” by the Portsmouth North MP’s announcement.
“Penny has the experience, the compassion and the determination to lead our country to a bright future!” the former cabinet minister said.
Born in Torquay in Devon, the 49-year-old Ms Mordaunt is the daughter of a paratrooper and a special needs teacher.
She worked on George W Bush’s presidential campaigns and was a Tory party staffer during William Hague’s leadership, later making headlines for her appearance on the reality TV diving show Splash in 2014.
