Secret UK diplomatic cables from its ambassador to the US describe President Donald Trump as "inept," "insecure" and "incompetent".
The leak of the cables, published in the Daily Mail and confirmed to CNN by a UK government official, risk angering Trump and causing serious diplomatic damage to the allies' relationship.
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Ambassador Sir Kim Darroch was candid in his assessment in the cables, warning the White House was "uniquely dysfunctional" and Trump's career could end in "disgrace".
"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 wrote in one document.
Sir Kim also warned Trump could have been indebted to "dodgy Russians" and his economic policies could wreck the world trade system.
The White House told CNN it has no comment on the story, however, the leak is embarrassing for the British Government.
"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," the British Foreign and Commonwealth Office said in a statement.
"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 that these will withstand such mischievous behaviour."