Former Tory leadership contender Penny Mordaunt has endorsed Liz Truss in the race to be prime minister, in a huge blow to Rishi Sunak.

Ms Mordaunt announced her backing for the foreign secretary as she introduced her at the latest Conservative leadership hustings in Exeter, declaring Ms Truss as “the hope candidate” who “knows what she believes in”.

She added that Ms Truss would “clobber our opponents”.

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Ms Mordaunt was the final candidate to be knocked out of the contest last month in the fifth round ballot, leaving Ms Truss and Mr Sunak to go head-to-head for Conservative Party members’ votes.

The trade minister and former defence secretary came second in all previous rounds of voting, but a late surge by Ms Truss – who gained 27 votes in 24 hours – cost her a place on the ballot, with only eight votes separating them.

Addressing the audience, Ms Mordaunt said: “This isn’t just about one person.

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“This is about all of us, and I know it’s not easy, you feel huge responsibility. I do too.

“That is the price, I’m afraid, of democracy. If we get this decision wrong, we’ll short change the country.

“We may lose an election and everything that we have worked for will be gone. That’s why this matters so much – why you feel that responsibility.”

She added: “I could have remained undeclared, I could be off sipping pina coladas right now, but I am not, I am here with you because this is too important.

“And I am not going to leave this to chance, and nor will you. I came third in this contest, and I owe it to all of you to be a signpost, not a weathervane.

“And so I’ve made my choice – I know it’s difficult because I like both candidates.

“I want to say nice things about both of them, which is probably why I came third.

“I’ve seen enough to know who the person I’m going to put my faith in is. And that is Liz Truss.”

Making her entrance on to the stage, Ms Truss thanked Ms Mordaunt for her support.

“It’s fantastic to have the support of Penny Mordaunt,” she said.

“She’s a great person, a great politician and a great patriot – and I am proud to call her my friend.”

Chief political correspondent

It’s the endorsement both candidates desperately wanted in the battle for the Tory crown. Potentially the biggest vote winner so far, declaring for Liz Truss.

Penny Mordaunt was runner-up to Rishi Sunak in all but the final round of voting by MPs, before being narrowly edged out by Liz Truss in the nail-biting 11th-hour showdown.

In round one she won 67 votes to 50 for Ms Truss, in round two it was 83-64, round three was 82-71 and round four 92-86, before she finally polled just 105 to 113 for Ms Truss.

Now, in a shock move after the bad blood between their two camps during the MPs’ voting, the former magician’s assistant has sprinkled her stardust on the Liz Truss campaign.

In a stunning piece of theatre at the Tory Party’s hustings at the University of Exeter, she dramatically climbed on board the Truss bandwagon and almost certainly earned herself a cabinet job if the foreign secretary defeats Mr Sunak.

Cynics will claim Ms Mordaunt’s endorsement of Ms Truss would have carried more weight had it come earlier.

But the timing – the day ballot papers were being sent to party members – is ideal for Ms Truss and a crushing blow to Mr Sunak.

Fellow former leadership candidate Tom Tugendhat also endorsed Ms Truss last week.

Writing in The Times, Mr Tugendhat said that the foreign secretary’s promises of tax cuts are based on “true Conservative principles” and that she can unite the party.

He said: “Liz has always stood up for British values at home, and abroad. With her at the helm, I have no doubt that we will move with determination to make this country safer and more secure.”

Mr Sunak was initially the frontrunner in the race, winning the most support from MPs in the first few rounds of voting.

But polling has shown Ms Truss is more popular among party members, who will ultimately decide who will be the leader and next prime minister.

The members’ ballot will close on 2 September, and whoever is named the winner on 5 September is expected to replace Mr Johnson as prime minister the following day.

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