LONDON: Kim Darroch is a superb diplomat and public servants should not be dragged into the political agenda, Boris Johnson, the frontrunner to become prime minister, said on Wednesday after Britain's ambassador to the United States resigned in a furore over leaked memos.

Darroch quit after Donald Trump labelled him "stupid" and "wacky" following the leaking of the memos in which he branded the US president's administration inept.

Johnson, a former foreign minister, repeatedly failed to back Darroch during a televised debate late on Tuesday.

But during a campaign visit on Wednesday, Johnson said of him: "he is a superb diplomat," adding that whoever leaked the documents "really has done a grave disservice to our civil servants, to people who give impartial advice to ministers."

"It is not right that civil servants' careers and prospects should be dragged into the political agenda," Johnson said.