In latest tweets on Kim Darroch, US president also attacks Theresa May for making a mess of Brexit

Donald Trump has launched a scathing attack on Theresa May and said the US would no longer deal with the British ambassador to Washington after the diplomat’s frank assessments of the president as “inept” and “dysfunctional” were leaked to the Mail on Sunday.

In a sign of the damage to the US-UK relationship, Trump hit out at Sir Kim Darroch for a second day in a row, threatening to cut ties over the memos that described him as “radiating insecurity”.

The US president also said May and her representatives had made a “mess” over Brexit, contrary to his advice. Just weeks ago, he praised the prime minister in person on his state visit to the UK, saying she had done a “very good job”.

Trump launched a tirade against Darroch, saying: “I do not know the Ambassador, but he is not liked or well thought of within the U.S. We will no longer deal with him. The good news for the wonderful United Kingdom is that they will soon have a new Prime Minister. While I thoroughly enjoyed the magnificent State Visit last month, it was the Queen who I was most impressed with!”

Donald J. Trump (@realDonaldTrump) ....thought of within the U.S. We will no longer deal with him. The good news for the wonderful United Kingdom is that they will soon have a new Prime Minister. While I thoroughly enjoyed the magnificent State Visit last month, it was the Queen who I was most impressed with!

The tweets are a sign that Trump’s displeasure with Darroch has intensified since he told reporters on Sunday: “We are not big fans of that man and he has not served the UK well … so I can understand it and I can say things about him, but I won’t bother.”

His latest statement creates a dilemma for the new prime minister, likely to be Boris Johnson, who will have to decide whether to replace Darroch and risk looking as if he has been bullied into the action by Trump. However, keeping him in post risks further damage to the UK’s diplomatic relations with one of its closest international allies.

Liam Fox, the international trade secretary, was in Washington on Monday to attempt to smooth over relations and was due to apologise for the leak to Ivanka Trump, the president’s daughter and adviser.

He told the BBC: “This is such a damaging, potentially damaging event that I hope the full force of our internal discipline, or even the law, will come down on whoever actually carried out this particular act.”

Jeremy Hunt, the foreign secretary and Johnson’s rival for the Tory leadership, also tried to limit the damage by saying he did not agree with all the views expressed by Darroch, and insisted that Britain had the “warmest” of relationships with the US.

Play Video 0:33 Donald Trump: 'We're not big fans' of UK ambassador to US – video

However, May’s official spokesman also expressed “full faith” in Darroch and defended his right to make an “honest and unvarnished” assessment of the White House, saying he should not be sacked from his role, even though May disagreed with his analysis. It is this qualified defence of Darroch from Downing Street that appears to have further infuriated the president.

Following Trump’s tweets, No 10 reiterated that May would not be sacking Darroch in her final two weeks as prime minister.

A government spokesman said: “We have made clear to the US how unfortunate this leak is. The selective extracts leaked do not reflect the closeness of, and the esteem in which we hold, the relationship.

“At the same time, we have also underlined the importance of ambassadors being able to provide honest, unvarnished assessments of the politics in their country. Sir Kim Darroch continues to have the prime minister’s full support.”

It is not clear from Trump’s tweets whether he personally is withholding cooperation from Darroch, or if he will instruct the entire US administration, including White House staff, the national security adviser and the state department, to cut ties with the ambassador.

If such an explosive move were to take place, it would effectively render Darroch persona non grata in Washington and of no use as an interlocutor between the British and American governments.

The effect would be to increase the pressure on Downing Street to pull Darroch out of Washington earlier than planned. However, that may be resisted by some Foreign Office officials on the basis that he had done nothing wrong and his departure would be a victory for those who chose to leak his telegrams. In practice, a government can reject an ambassador, although they cannot choose the identity of the person holding the position.

Alternatively, Darroch may recognise that the doors are shutting in his face and that there is nothing to be gained by staying, which might put strain on the so-called special relationship.

The leak has caused outrage among MPs, who circulated theories in parliament that the culprit could be Russian government hackers or another hostile state.

Isabel Oakeshott, the journalist who received the leak, is close to a number of pro-Brexit politicians and co-wrote an account of the EU referendum with Arron Banks, the leave-supporting donor, who is under investigation by the National Crime Agency over the sources of funding for the Leave.EU campaign.

However, Alan Duncan, a Foreign Office minister, told the Commons it was the government’s belief that the leak made its way to Oakeshott “from within” Whitehall, rather than being handed over by foreign agents. He said the Cabinet Office had begun its formal hunt for the mole among current senior politicians, aides and civil servants, but did not at this stage think it was right to involve the police.

He said one of the documents was saved from 2017, but three of the memos were produced just eight to 10 days ago, which appeared to point to someone who was still in post.

The cross-government inquiry has already begun examining who had access to the recent memos – which went to potentially hundreds of people – and a highly sensitive letter from 2017 that had a much narrower circulation list.

Tom Tugendhat, the Conservative chairman of the Commons foreign affairs committee, told MPs he had written to the Metropolitan police commissioner, Cressida Dick, to “ask that a criminal investigation also be opened into the leak”.

Downing Street is resisting demands that the police get involved and insisted it would call in the Met only if the inquiry found evidence of criminality. A similar investigation into who leaked a National Security Council decision about the Chinese company Huawei led to Gavin Williamson losing his post as defence secretary.

In the diplomatic memos, it was suggested “you need to make your points simple, even blunt” to communicate with Trump.

Darroch also gave a scathing assessment of the White House, saying: “We don’t really believe this administration is going to become substantially more normal; less dysfunctional; less unpredictable; less faction-riven; less diplomatically clumsy and inept.” He questioned whether the White House “will ever look competent”.

Ned Price, who served under Barack Obama as a special assistant to the president for national security affairs, said Trump’s tweets only reinforced Darroch’s descriptions of him in private.

“The UK ambassador didn’t write anything that wasn’t already patently obvious to an informed observer,” Price said. “He spoke of Donald Trump as someone with thin skin, as someone with a delicate ego, as someone who would be receptive to pandering, and just look at Trump’s reaction.

“He has proved the ambassador right in every respect — by proving he is ego driven, by proving he is thin-skinned, and by proving he will quite literally refuse to deal with people who don’t say nice things about him.”