US President Donald Trump has hit out at the British ambassador there after a leak of sensitive memos, saying he "has not served the UK well".
In the memos, obtained by the UK's Mail on Sunday, Kim Darroch described the US president as "incompetent", "inept" and "insecure".
He also gave a scathing assessment of the White House, saying it had become "uniquely dysfunctional" under Mr Trump because of "vicious infighting and chaos", and questioned whether it "will ever look competent".
A formal inquiry will be held into the leak of the diplomatic memos.
Officials have insisted the relationship with the White House can withstand the "mischievous behaviour" of the leak and defended Mr Darroch's candid style.
Asked about the controversy in New Jersey, Mr Trump told reporters: "The ambassador has not served the UK well, I can tell you that.
"We're 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."
Britian's Foreign Secretary Jeremy Hunt has distanced himself from the remarks, saying: "This was a personal view."
He added: "It's not the view of the British government. It's not my view.
"We continue to think that under President Trump the US administration is not just highly effective but the best possible friend of the United Kingdom on the international stage."
The memos, which detail Mr Darroch's assessments of the Trump administration from 2017 to the present, suggest that in order to communicate with the president "you need to make your points simple, even blunt".
Following Mr Trump's state visit to the UK in June, Mr Darroch said that although the president had been "dazzled" by the pomp and ceremony of the trip, his administration would remain self-interested and "this is still the land of America First".
In one of the most recent documents, he refers to "incoherent, chaotic" US policy on Iran and questions Mr Trump's publicly stated reason for calling off a retaliatory airstrike against Tehran following the downing of a US drone.
The US and Iran have been at the brink of armed conflict over tensions in the Gulf, and Mr Trump stated that he called off a planned airstrike with minutes to spare because of the potentially high number of casualties.
But Mr Darroch said that 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," he 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".
Referring to early allegations of collusion between the Trump campaign and Russia, the memo said "the worst cannot be ruled out".
An investigation by Robert Mueller published earlier this year cleared the Trump team of the collusion claims.
The former British ambassador to the US, Peter Westmacott, said the leak had put Mr Darroch in an "embarrassing position".
Writing in Britain's Daily Telegraph, he said: "Perhaps there are people at the heart of government - the distribution of the published material was, after all, very limited - who think this will persuade the incoming prime minister to appoint in his place someone less ready to speak truth to power than Sir Kim has been; or someone already regarded as a 'friend' by the Trump administration.
"Giving sensitive papers of this kind to the media is the biggest possible disincentive to ambassadors to say what they really think to ministers back home."