Conservative Party leadership hopeful Jeremy Hunt has refused to commit to taking Britain out of the European Union before Christmas, saying prime ministers should "only make promises they know they can deliver".

The Foreign Secretary insisted it was possible to get a revised deal with Brussels by the end of September, and said if it took "a few extra days" to get it through parliament he would delay Brexit beyond the 31 October deadline.

But, when pressed during an interview with the BBC's Andrew Neil, he refused to say whether the UK would have left by Christmas, though claimed "it's not going to be months".

Mr Hunt has said that he believes he would be able to get a new deal with Brussels, but if that proved impossible, he would prepare for no deal on 31 October, making a judgment on the best course to follow at the end of September.

He expressed his confidence in getting a deal by the end of September, telling the BBC: "I believe we can and I, as I say, I think that people like Angela Merkel want to solve this problem.

"If we have a deal, if it's clear to us and to the Europeans there's a deal to be done, then of course I would go for that and if it took a little bit - you know, a few extra days - to get it through parliament."

He said parliament would be "willing to sit at weekends, will be willing to sit late, to do this" but that it "may take a few extra days and I would be willing to allow those days".

Asked whether he would be prepared to delay by days, weeks or months, Mr Hunt replied: "Well it's not going to be months."

But pressed on whether the UK would be out by Christmas, he said: "I'm not going to give you those commitments.

"It's because prime ministers should only make promises they know they can deliver. And there's another reason why we have to be careful about this 31st of October date.

"It is because parliament may try and take a no-deal Brexit off the table altogether and so I think - my commitment is that I think I'm the best person to get a deal and if we get a deal it will be on or around the 31st of October but I can't control what Parliament does and that's why I'm being honest with people about the difficulties."

In a swipe at his rival Boris Johnson, Mr Hunt said he would deliver Brexit "more quickly than the alternative".

"If you want to leave the EU quickly, if you want to avoid a general election, which is the risk if you go about this in the wrong way, I'm the person who has the biggest chance of negotiating that deal and getting us out by 31 October."

In his interview with Mr Neil, Mr Johnson said the EU will agree a Brexit deal this year without the backstop provision, predicting the EU would instead negotiate the issue during post-Brexit free trade talks.

"We will get a deal by October 31st," Mr Johnson said.

"They (the EU) need to take ... the Irish backstop and they need basically to remit it, to remove it, to delete it," he added.

Mr Johnson said it was "manifestly in the interests of both sides" to reach a new deal, noting "the EU has a very substantial net balance of trade with us".

"They will want to continue to see those goods flowing," he added.

Mr Johnson also admitted a report of his comments about Kim Darroch was a "factor" in his resignation as Britain's ambassador to the United States.

However, the Tory leadership frontrunner denied failing to give Mr Darroch his backing and said his words during a televised leadership debate had been "misrepresented".

Mr Johnson's lack of explicit support for Mr Darroch during the ITV debate on Tuesday was widely seen to have been the final straw for the envoy following the leak of his diplomatic cables criticising Donald Trump's White House.

Furious Tory MPs accused the former foreign secretary, who is favourite to succeed Theresa May, of throwing Mr Darroch "under a bus", leaving him no option but to resign.

But in tonight's BBC interview, Mr Johnson said when he spoke to Mr Darroch by telephone the ambassador told him he had not seen the debate himself although somebody had told him about Mr Johnson's comments.

"He said that what somebody had relayed to him had been a factor in his resignation," Mr Johnson said.

He added: "I think that unfortunately what I said on that TV debate was misrepresented to Kim."

The Metropolitan Police said this evening it had opened a criminal investigation into alleged leaking of official communications Mr Darroch.

Elsewhere, British Prime Minster Theresa May has said she will leave Number 10 with a mixture of pride and disappointment, as she admitted underestimating the "entrenched positions" in parliament over Brexit.

Mrs May conceded she could have talked to more colleagues to get her withdrawal deal through the House of Commons.

She said if the future relationship had been negotiated alongside there would not have been the same issue with the backstop.

In an interview with the BBC ahead of her departure from Downing Street on 24 July, Mrs May was asked how she thought she would feel when she walks out of the door of Number 10 as prime minister for the last time.

She said: "I think it'll be a mixture of pride at having done the job. But also a degree of disappointment because there was more that I wanted to do.

"I think we have achieved a lot over the last three years but whenever you come to the end of a premiership I think everybody will always feel that there is more that they wanted to do."

Mrs May said she had underestimated the unwillingness of some people in parliament to compromise to deliver Brexit.

"On the one hand, some people who'd always campaigned for Brexit, but didn't vote for the deal because they had a particular vision of Brexit and they were sticking firmly to that vision.

"On the other side, people who said they didn't want to leave with no deal, but weren't prepared to vote for a deal in order to make sure that what they wanted happened.

"So I think that I underestimated the unwillingness of parts some people in parliament to compromise."