Boris Johnson, the favorite to succeed Theresa May as U.K. prime minister, said the departure of Britain's ambassador to the U.S. has been politicized as he sought to deflect criticism of his reaction to the transatlantic diplomatic spat.

The envoy, Sir Kim Darroch, quit Wednesday after leaked diplomatic cables in which he wrote about Donald Trump in unflattering terms sparked a volley of tweeted insults from the U.S. president.

Johnson's failure to defend Darroch in a televised debate Tuesday was a factor in the envoy's resignation, a Foreign Office official told Bloomberg. That was in stark contrast to Johnson's leadership rival, Foreign Secretary Jeremy Hunt, who said Trump's outburst had been "disrespectful and wrong." Alan Duncan, a minister in the Foreign Office, said in the wake of the debate that Johnson had thrown Darroch "under a bus" by refusing to back him.

"There has certainly been an attempt to politicize this issue and to take the career prospects of Sir Kim and turn them into an issue in the Conservative Party leadership contest," Johnson said late Thursday in a web-cast leadership hustings in Maidstone, southeast England. "I will stand up for our fantastic diplomats across the world, I just don't think that their careers should be used as political footballs."

Asked twice, Johnson failed to reply why he hadn't defended Darroch on Tuesday, saying only "I give him my full support."