A formal investigation is to be launched into leaked memos describing Donald Trump as "uniquely dysfunctional" and "inept".

Britain's Ambassador to Washington Sir Kim Darroch is facing calls to be sacked over the disclosure issue. He reportedly impugned the US president's character in a series of cables and briefing notes.

Sir Kim warned that under Mr Trump, the White House has become "uniquely dysfunctional" and dogged by internal conflict, according to the Mail on Sunday, who claim to have seen the leaked memos.

Sir Kim is also reported to have said that the US leader could have become indebted to "dodgy Russians" during his presidency.

The claims risk angering Mr Trump and could undermine the UK's "special relationship" with the US.

A Foreign Office spokesman said a formal leak investigation will now be initiated. A spokeswoman added: "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.