Boris Johnson refuses to rule out sacking UK ambassador to the US Sir Kim Darroch, after Trump described him as a "very stupid guy" and refused to deal with him.

The frontrunner to replace Theresa May as prime minister said he would not be so "presumptuous" as to say Darroch would keep his job.

He also refused to criticise Trump after the president tweeted a series of criticisms of Theresa May and her handling of Brexit.

Johnson made the comments as part of a televised Conservative leadership debate with his rival Jeremy Hunt on Tuesday evening.

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LONDON — Boris Johnson has put the future of the UK's ambassador to the US in doubt after Donald Trump labeled him a "very stupid guy" and said he would no longer deal with him.

Johnson, who is the favourite to replace Theresa May as prime minister at the end of July, refused repeated requests by his rival Jeremy Hunt to say that he would keep Sir Kim Darroch in his diplomatic post.

Johnson, who worked closely with Darroch while he was Foreign Secretary, told the ITV Conservative leadership debate on Tuesday evening that he "wouldn't be so presumptuous," as to say whether he should keep his job.

Trump's tweets led to condemnation from Johnson's rival, Jeremy Hunt, who labeled them "disrespectful and wrong."

However, Johnson refused to criticise Trump on Tuesday evening, saying only that the president had been "dragged into a British political debate."

A senior ally of Mr Johnson told the Times: "We don't want to put the whole special relationship on the edge because of a row about one person. We do not have anything to gain from a running spat with the White House."

The exchange came after the president escalated a diplomatic row with the UK on Tuesday, calling Prime Minister May's handling of Brexit "foolish" and a "disaster," and launching a new attack on Darroch, labeling him a "very stupid guy" and a "pompous fool."

Trump's outburst followed the leak of confidential memos by Darroch to the UK government, which labeled Trump's administration "uniquely dysfunctional," "incompetent" and "inept."

Read more: The UK is investigating whether Russian spies hacked secret memos branding Trump 'uniquely dysfunctional' and 'incompetent'

Read more:Police asked to investigate explosive UK diplomatic leak labeling Trump's White House 'uniquely dysfunctional' and 'incompetent'

Darroch was subsequently disinvited to an upcoming US banquet, with Trump insisting that he would refuse to work with him any longer.

Trump on Sunday told reporters in New Jersey that Darroch "has not served the UK well, I can tell you that" and adding: "We are not big fans of that man and he has not served the UK well.

Watch Boris Johnson refuse to rule out sacking US ambassador

The UK government insisted on Tuesday that it would be standing by Darroch.

"The United Kingdom Government determines who its ambassador is," a spokesman for May said following Trump's comments.

"Sir Kim continues to be our ambassador, we have full confidence in him in that role."

They added: "The PM has made clear to Sir Kim that he has her full support."