Penny Mordaunt is the first MP to declare she will be standing to be the next Conservative Party leader and Prime Minister.

Mordaunt, the leader of the House of Commons under Liz Truss, came third in September behind Rishi Sunak.

Possible contenders have been gauging their support among MPs since Truss announced she was quitting yesterday.

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Mordaunt said: “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.”

Speculation has been mounting that Boris Johnson, the former Prime Minister forced to resign in the summer, will stand again if he has enough support.

Close ally Jacob Rees-Mogg, business, energy and industrial strategy minister, has already backed Johnson.

Sunak, the former chancellor beaten by Truss in the last contest, is expected to stand again and is understood to have the most support among MPs, so far.

He was the choice of MPs during the previous contest before the membership opted for Truss.

Ben Wallace, defence secretary, has ruled himself out and said he is “leaning towards” backing Johnson.

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He added: "This will be potentially our third Prime Minister since the General Election of 2019. That means we have to think about that legitimacy question that the public will be asking themselves, and also about who could win the next election - that's obviously important for any political party at the time."

MPs have until Monday to get 100 nominations from fellow MPs and go into the ballot stage of the contest.

The threshold means with 357 Tory MPs there can be a maximum of three MPs in contention.

If there are three candidates, MPs will vote for their preference and one will be knocked out and the remaining two will go to a vote of the party members with a result declared on Friday, October 28.

If there are two, MPs will still vote to give members an indication of who the parliamentary group supports.

If only one gets the required 100 nominations, then they will be elected unopposed without a vote among members and take over as leader on Monday.

MPs will vote between 3.30pm and 5.30pm on Monday.