LONDON — In a series of leaked diplomatic cables, Britain’s ambassador to the United States, Kim Darroch, described President Trump as “radiating insecurity” and his administration as diplomatically “clumsy and inept,” a withering assessment that threatened to damage bilateral relations at a delicate moment for Britain.

The cables were published late on Saturday by The Mail on Sunday, span a period from 2017 to the present and include candid assessments of American domestic politics and Washington’s treatment of Iran over its nuclear weapons program.

The most closely held of the cables was intended as an update on the new Trump administration for a narrow audience of top British officials. It described the chaos inside the new administration, the concerns about the future of the Atlantic relationship and the struggle to figure out who had the president’s ear.

It was unclear who leaked the documents and how The Mail obtained them. But the British news outlet identified only one recipient in Britain: Mark Sedwill, a longtime British diplomat who succeeded Mr. Darroch as national security adviser and became cabinet secretary in 2018.