Theresa May has described the resignation of the US ambassador as "a matter of great regret" amid a furious backlash over Boris Johnson's refusal to support him.

Sir Kim Darroch said his job had become "impossible" after the leak of diplomatic memos highly critical of Donald Trump's administration.

His decision is understood to have been made after Mr Johnson repeatedly refused to support him during a televised Tory leadership debate on Wednesday.

Foreign office minister Sir Alan Duncan has absolutely torn into Boris Johnson over the resignation of Sir Kim Darroch.

Asked if Johnson should have defended the ambassador, Sir Alan said: "Yes he should have done so. The current foreign secretary Jeremy Hunt gave him full support, so did the PM, so did the cabinet.

"A former foreign sec who thinks he that is equipped to be PM has basically thrown him under the bus.

"MPs on both sides of the house are absolutely livid and I think the conduct of BJ has discredited him enormously in the eyes of many people in this country."