Downing Street has said Prime Minister Theresa May has "full faith" in the UK's ambassador to the US after leaked emails showed Sir Kim Darroch believed Donald Trump's administration to be "inept".
The extraordinary breach of confidentiality triggered a political firestorm on both sides of the Atlantic as Mr Trump made clear his anger at Sir Kim.
While showing support for the ambassador and the need for "unvarnished assessments" of foreign political situations, Number 10 distanced itself from Sir Kim's withering assessment of the Trump White House.
The Prime Minister's official spokesman told a Westminster briefing: "The PM has full faith in her ambassador to Washington.
"Our ambassadors provide honest, unvarnished assessments of politics in their country – those views are not necessarily the views of ministers or indeed of the Government.
"As the Foreign Secretary has said, this leak is not acceptable.
"We would expect such advice to be handled in the correct way and a leak inquiry has been launched."
Asked if Mrs May agreed with the contents of Sir Kim's leaked assessment of the Trump administration, the spokesman said: "The PM does not agree with that assessment.
"The Prime Minister has a good relationship with the president and the Government works closely and constructively with the administration across a wide range of issues."
The spokesman said there was "nothing to suggest" hostile state actors had been involved in the leak.
Number 10 said officials had contacted the Trump administration and set out that they believe it is "unacceptable".
"The leak is absolutely unacceptable and as you would expect contact has been made with the Trump administration setting out our view that we believe that it is unacceptable," the spokesman said.
"It is a matter of regret that it has happened."
The comments came after International Trade Secretary Liam Fox called for the law to be involved in how the explosive and supposedly secret remarks from the British ambassador to the US became public.
Mr Fox expressed concern the leaks could damage the UK's relationship with Washington after President Trump made clear his displeasure with Britain's chief envoy to the US.
Mr Trump hit out at the UK's ambassador to Washington after the leak of sensitive diplomatic messages painting an unflattering assessment of his administration.
The president said Sir Kim had "not served the UK well" and his administration were "not big fans" of the envoy.
Mr Fox, who is visiting the US, 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."
A formal investigation will be carried out to determine how the candid and highly embarrassing messages from Sir Kim to the UK Government were leaked.
Mr Trump – who was described as "radiating insecurity" in Sir Kim's assessment – was clearly annoyed by the leak.
He told reporters: "The ambassador has not served the UK well, I can tell you that.
"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."
Foreign Secretary Jeremy Hunt said he did not agree with all the views expressed by Sir Kim and insisted Britain has the "warmest" of relationships with the US.
Speaking at a press conference at the Foreign Office, Mr Hunt said: "It's a personal view and there will be many people in this building who don't agree with that view and indeed I don't agree with some of the views that we saw in those letters.
"I think the US administration is highly effective and we have the warmest of relationships and a partnership based on standing up for shared values."
Mr Hunt said the leak was "not acceptable" and warned there would be "very serious consequences" if the culprit is identified.
"I'm very concerned about it because fundamental to the proper functioning of our diplomatic network, which I happen to believe is one of the finest, if not the finest, in the world, is to be able to exchange opinions frankly," he added.
The documents obtained by the Mail on Sunday detail Sir Kim's assessments of the Trump administration from 2017 to the present.
A Foreign Office spokesman said: "A formal leak investigation has now been initiated."
Officials insisted the relationship with the White House could withstand the "mischievous behaviour" of the leak.
The diplomatic memos suggest that in order to communicate with the president "you need to make your points simple, even blunt".
In the cache of documents, Sir Kim gives a scathing assessment of the White House: "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".
In one of the most recent documents, Sir Kim refers to "incoherent, chaotic" US policy on Iran and questions Mr Trump's publicly stated reason for calling off a retaliatory air strike against Tehran following the downing of an American drone.
The US and Iran have been at the brink of armed conflict over tensions in the Gulf and Mr Trump stated he called off a planned air strike with minutes to spare because of the potentially high number of casualties.
But Sir Kim said the explanation "doesn't stand up" and suggested it may have been motivated by Mr Trump's focus on the 2020 re-election campaign and his previous promises not to involve the US in foreign conflicts.
"It's more likely that he was never fully on board and that he was worried about how this apparent reversal of his 2016 campaign promises would look come 2020," Sir Kim said.
He said it was "unlikely that US policy on Iran is going to become more coherent any time soon" as "this is a divided administration".
Many people would like to see @Nigel_Farage represent Great Britain as their Ambassador to the United States. He would do a great job!
— Donald J. Trump (@realDonaldTrump) November 22, 2016
Brexit Party leader Nigel Farage, who Mr Trump has previously touted as a possible British ambassador to the US, said he would not be the right person for the job.
He told BBC Radio 4's Today programme: "Well, I'm not a diplomat, and I think that's quite an understatement."
Pressed on whether he would take the post of ambassador to the US, Mr Farage said: "No. I don't think I'm the right man for that job."