Boris Johnson has conceded Sir Kim Darroch's decision to quit his role as UK ambassador to Washington was in part influenced by his comments in the ITV debate.
The Conservative leadership candidate said his views were "misrepresented to Kim".
The former foreign secretary faced an outburst of criticism in the aftermath of the debate in which he appeared to lack support for the diplomat.
Speaking to the BBC's Andrew Neil, Mr Johnson said: "He said that what somebody had relayed to him had certainly been a factor in his resignation. I think that unfortunately what I said on that TV debate was misrepresented to Kim."
In the fiery interview, Mr Johnson added he had spoken to Sir Kim following the debate and had expressed sadness at the news he had decided to vacate the role.
Quizzing his fellow candidate on the ambassador's position, Mr Johnson said he "would not be so presumptuous" as to say he would keep him in the role after he is due to leave it at the end of this year.
Analysis: Libby Wiener explains Mr Johnson's diplomatic debarcle
Leadership rival, Jeremy Hunt, said he was "disappointed" by the news of the UK's man in Washington leaving the role.
He added: "I think we have to back our diplomats all over the world.
"Sir Kim was doing his job. He was giving his own personal but totally honest view about the country he was serving in."
The Metropolitan Police has launched an investigation into the leak, with officers from Counter Terror teams looking into whether the Official Secrets Act has been broken.
Following the interviews, Mr Johnson was heckled during hustings in Cheltenham.
An audience member shouted: "Answer the bloody question", as Mr Johnson was grilled about the resignation of Sir Kim Darroch.
Hunt refuses to name date for UK to leave EU if he becomes PM
Jeremy Hunt has refused to commit to taking the UK out of the European Union by Christmas if he becomes prime minister.
The foreign secretary said whilst it is his intention to deliver Brexit by 31 October, it may not be possible to get legislation through parliament by that date.
Taken to task on the issue by Andrew Neil, Mr Hunt said he wouldn't name a date because "prime ministers should only make promises they know they can deliver".
He said Parliament would be "willing to sit at weekends, will be willing to sit late, to do this" but that it "may take a few extra days and I would be willing to allow those days".
Asked whether he would be prepared to delay by days, weeks or months, Mr Hunt replied: "Well, it's not going to be months."
Mr Johnson repeated his stoic stance the UK should leave the European Union on 31 October regardless of a deal.