Rishi Sunak was under fire on Friday as the two contenders bidding to become Britain’s next Prime Minister prepared for a crucial weekend of campaigning.

With many of the 160,000 Conservative party members expected to vote soon after receiving their postal ballots early next week, the former chancellor was dealt a blow when Defence Secretary Ben Wallace announced that he was endorsing Foreign Secretary Liz Truss.

Doubling down on criticism of Mr Sunak , Mr Wallace questioned his experience for the top job and said he had been wrong to resign from the Government earlier this month, triggering Boris Johnson’s resignation.

Liz Truss speaking at an event at Breckland Council in Dereham, Norfolk, as part of her campaign to be leader of the Conservative Party and the next prime minister

Explaining his decision to back Ms Truss on Sky News, Mr Wallace said: “She’s actually incredibly experienced in government. She is the only candidate that did nearly two years in the Treasury as the Chief Secretary to the Treasury. She was in international trade, and negotiated trade deals. And now she has been a Foreign Secretary. That’s a broad experience that I’m afraid Rishi doesn’t have. Yes, he’s been in the Chancellery, but he doesn’t have that broad experience.”

Before the Tory leadership contest began earlier this month, the Defence Secretary was seen as a frontrunner to replace Mr Johnson, coming out on top in early polls of Conservative party members who will ultimately make the decision by September 5. But despite winning plaudits for his handling of Russia’s invasion of Ukraine, Mr Wallace decided not to run, sparking a tussle among candidates to secure his support.

Following Thursday night’s first hustings event for the two candidates in Leeds, they were back on the campaign trail on Friday with the former chancellor expected to canvass Tory members in Kent while the Foreign Secretary was heading to Suffolk and Norfolk.

One senior Tory figure said Ms Truss had beaten expectations during the Leeds debate with a more polished performance and a focus on local issues which played well with the audience.

Former cabinet minister Greg Clark (L) introduces Rishi Sunak, candidate to become Britain's next prime minister and Conservative party leader, during a campaign event in Tunbridge Wells, Ken

But while she is enjoying a comfortable lead in early polls of Tory members, the senior Tory said Mr Sunak’s pitch to the wider electorate may convince some undecided voters that he represents the best chance of securing a general election victory over Labour.

A YouGov poll earlier this week showed that Mr Sunak is more popular with swing voters than Ms Truss but among Tory members Ms Truss is way out in front, partly due to her promises to cut taxes immediately but also because Mr Sunak is seen as disloyal for quitting.

In an appeal to the Tory base, Mr Sunak vowed on Thursday to double the number of foreign criminals deported from the UK if he becomes Prime Minister. He said: “We are far too soft on foreigners who commit crimes in our country so I will double the number of foreign offenders we deport. I will cut crime by locking the most prolific offenders up, keeping them locked up, and building the prison space needed to do so.”

But after Tory members at the Leeds hustings questioned Mr Sunak over his decision to resign as chancellor, Mr Wallace said: “I just don’t think walking out at a time of a crisis is the right course of action. There were other mechanisms to do what they wanted. If Rishi Sunak didn’t want the Prime Minister to be Prime Minister, there are other mechanisms to do that.

“And that goes for all the other ministers. He made his choices. He reconciles himself with that. My judgment was, you know, well, first of all, I didn’t want the Prime Minister to go but if I had been in that position, I think there was another mechanism of doing it.”

Former security minister Damian Hinds, who is backing Mr Sunak, defended his decision to quit, citing major differences with Mr Johnson on economic policy and insisted the former chancellor could still win the contest.

He added: “The more members hear from Rishi, the more they get to meet Rishi actually, the more inclined they are to vote for Rishi as the candidate with the record, the experience, the character and the vision, not only to put in place, Conservative policies to make sure that we can win this historic fifth general election that we need to win against Labour.”

Mr Hinds also defended Mr Sunak’s plans to cut VAT on energy bills and questioned Ms Truss’s tax cutting plans, which his camp argues will drive up inflation. He said there was a “big difference” between the plan to cut VAT on the bills and “unfunded, massive increases in spending or cuts in taxes and all the borrowing that brings with it”.