British politician Boris Johnson on Tuesday declined to back Kim Darroch, the British ambassador to the United States who in leaked emails referred to President Donald Trump as "inept" and his White House "dysfunctional."

When asked whether he would allow Darroch to keep his job should he become prime minister, Johnson said: "I'm not going to be so presumptuous as to think I'm going to be in a position (to do that)."

Johnson, the forerunner to succeed Theresa May, then said Trump had been dragged into a "British political debate."

"I don't think it was necessarily the right thing for him to do," he said in an ITV television debate. "But let's face it, our relationship with the U.S. is of fantastic importance.

"I have a very good relationship with the White House. It's very important that we should have a close friendship, a close partnership with the United States."

May supports Darroch staying in his current role despite criticism from Trump, who called the envoy "wacky" and a "very stupid guy."

• Boris Johnson: UK Immigrants Should Learn English
• Boris Johnson And Jeremy Hunt Stress Readiness to Quit EU Without a Deal