Boris Johnson under fire from rivals over his failure at full scrutiny and his 'hard-stop-at-any-cost' Brexit

Mr Stewart, the International Development Secretary, warned that Mr Johnson's reluctance to submit to media scrutiny was undermining trust in British politics.

"How is Boris going to deliver Brexit? He keeps saying 'I am going to deliver it'. I don't even know what he believes. He won't talk to me. He won't talk to you. He won't talk to the public," he told BBC1's Andrew Marr Show.

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The Scot went on: "We want to know what he believes. The real problem with politics is a problem of trust.

"Nobody has had the chance to question him and as soon as you question him and as soon as I sit down with him and ask the big question: how, how are you going to deliver Brexit? How are you going to get a no-deal through? Then it begins to come off the rails.”

Mr Stewart added: "We are a moderate country, we are not a Trumpian country."

Mr Hunt, in a thinly-veiled attack on his predecessor at the Foreign Office, said he would not commit to a "October 31 hard-stop-at-any-cost" Brexit if he won the race to succeed Theresa May.

However, he refused to put a timeframe on any further Brexit delay, saying no candidate could "sensibly answer" the question of how long might be needed to secure changes to the deal struck with the EU.

The Secretary of State insisted it would be possible to negotiate a new deal with the EU27 because it would be prepared to listen to a UK that was offering “better choices” and that it would be willing to look at the “whole package,” which included the future partnership, that would do away with the need for an Irish border backstop.

“You approach them with the right PM, we can get the right deal through,” he declared.

Mr Hunt stressed that, having talked to European leaders, it was clear they wanted to resolve the Brexit problem.

"They say that if they were approached by a British prime minister, someone they were willing to deal with, who had ideas how to solve the Northern Irish border, they would be willing to re-negotiate the package.

"They are prepared to look at whether you could get much more detail onto the future relationship - potentially that could be legally-binding, let's see - so that you don't need a backstop. I would never pretend that this is going to be easy but nor is it impossible."

READ MORE: Scottish Tory MP warns Boris Johnson against trying to "subvert" leadership vote

Mr Hunt also claimed it could even be done by the deadline of October 31 but stressed it would be a mistake to commit to leaving the EU by that date.

"I am not committing to an October 31 hard-stop-at-any-costs[Brexit]. If you do make that guarantee and you go with the wrong approach, then you are committing us to nothing other than a hard Brexit, a no-deal Brexit," he declared.

Mr Johnson has declined to appear in tonight’s Channel Four event, claiming the number of candidates would make it too “cacophonous”. He has agreed to take part in a live BBC debate on Tuesday night following that day’s vote, which will eliminate at least one of the current six contenders. The former London Mayor will also subject himself to an MPs’ hustings tomorrow.

Meanwhile, Dominic Raab, appearing on Sky News’s Sophy Ridge on Sunday, warned that if the Tories failed to deliver Brexit, then they would be “toast” at a subsequent general election.

"The Conservatives cannot win an election unless we have delivered Brexit,” insisted the former Brexit Secretary.

Mr Raab, who has said he would leave with or without a deal, defended his refusal to rule out suspending Parliament in order to ensure it could not block Brexit; something his opponents have insisted would be unconstitutional and an outrage.

"What is really scandalous here is the way that people are trying to sabotage the will of the people and break their promises left, right and centre to get us out of the EU," he argued.

"The big mistake we made in these negotiations was taking no-deal off the table. When we start ruling things out, we only weaken our chances of getting a deal.

"All those candidates that are going weak at the knees and saying: 'I'm not sure about this and that'; they are sending a message to the EU that they can take us for a ride. We have had three years of that. It is time to get this done.”

The Surrey MP added: "We gave people a decision. Now Parliament is trying to steal it back away from them. When people voted, they voted to leave."

In other developments -

*David Mundell, the Scottish Secretary, who backed Matt Hancock, the UK Health Secretary, in the first round, has come out for Michael Gove, saying the Environment Secretary’s “track record in Government is of supporting Scottish interests and standing up for our United Kingdom”. Former candidate Esther McVey, the onetime Work and Pensions Secretary, is now backing Mr Johnson, while Tobias Ellwood, the Defence Minister, who also backed Mr Hancock in the first round, is now supporting Mr Stewart.

*Amber Rudd, the Work and Pensions Secretary, who is backing Mr Hunt, warned the numbers were “easily there” within the Conservative Party to bring down the Government in a confidence vote, should it lead a drive towards a no-deal Brexit.

*Nigel Farage, the Brexit Party leader, said he expected Mr Johnson if he won the leadership contest would, at best, only be able to deliver a "rehash" of the Prime Minister’s Withdrawal Agreement and that there appeared "little chance" of the Tories delivering on his pledge to take Britain out of the EU on October 31.

*Pro-Remain Scottish peer and former diplomat Lord Kerr, one of the authors of the Article 50 process, accused the Conservative leadership contenders of promoting Brexit “unicorns,” saying: “What alarms me most about the current Conservative Party leadership race is that fiction and fantasy are back and harsh facts again forgotten as new promises, no less unrealistic, are made.”

*Labour’s Hilary Benn said Parliament would try to block any new Conservative PM taking Britain out of EU without a deal.