Leaked memos give Sir Kim Darroch’s withering assessment of the Trump administration and could prove highly embarrassing for the Foreign Office. British officials insist the relationship with the White House could withstand the “mischievous behaviour” of the leak and defended Sir Kim’s candid style. But Mr Farage, a friend of Mr Trump who has spoken at some of his political rallies, said the attack proved Britain needed a new representative in Washington.

The Brexit Party leader said: “From the moment Trump was elected this man was the wrong person to be the British ambassador – a globalist in outlook, totally opposed to the Trump doctrine.

“The comments are wholly unsurprising but for him to speculate about Trump’s alleged involvement with Russia shows him to be totally unsuitable for the job and the sooner he is gone the better.”

In the cache of documents obtained by the Mail on Sunday, 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.”

Following Mr Trump’s state visit to the UK in June, Sir Kim warned 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, 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 that he called off a planned air strike with minutes to spare because of the potentially high number of casualties.

But Sir Kim 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.

Referring to the early allegations of collusion between the Trump campaign and Russia, the memo said “the worst cannot be ruled out”.

A lengthy investigation by Robert Mueller published earlier this year cleared the Trump team of the collusion allegations.

A Foreign Office spokesman said: “The British public would expect our ambassadors to provide ministers with an honest, unvarnished assessment of the politics in their country.

“Their views are not necessarily the views of ministers or indeed the Government.

“But we pay them to be candid. Just as the US ambassador here will send back his reading of Westminster politics and personalities.

“Of course we would expect such advice to be handled by ministers and civil servants in the right way, and it’s important that our ambassadors can offer their advice and for it remain confidential.

“Our team in Washington have strong relations with the White House and no doubt. These will withstand such mischievous behaviour.”