THE person who leaked the UK ambassador’s confidential messages should be “hunted down and prosecuted,” Boris Johnson has insisted.

The frontrunner for the Tory leadership – blamed by some for Sir Kim Darroch’s resignation – defended his decision not to rally behind the former UK ambassador to the US during this week’s TV head to head with his rival Jeremy Hunt.

During party hustings in Kent, Mr Johnson said: “I don't think that issues of personnel in our civil service should become footballs in political conversation."

He said Sir Kim was the victim of a "very unpleasant stunt" by the person who leaked his diplomatic dispatches.

"I rang Kim yesterday and said how much I regretted his resignation," he said.

Asked about suggestions he had thrown Sir Kim "under a bus", Mr Johnson replied: "There has certainly been an attempt to politicise this issue and to take the career prospects of Sir Kim and turn them into an issue in the Conservative Party leadership contest, I notice that. I don’t think that should happen.”

Earlier, asked if it was fair that some people were blaming him for Sir Kim's departure, the former London Mayor replied: "I get blamed for lots of things."

He stressed: “The real culprit in all this is the person who chose to leak confidential advice given by a great civil servant to the Government and to ministers. That is really corrosive of trust in the civil service. We need to find whoever did it, hunt them down and prosecute them."

In response to an Urgent Question in the Commons, Sir Alan Duncan, the Foreign Office Minister, said the leak inquiry was focusing on whether "someone within the system" was responsible for the leaking of the diplomatic cables.

Earlier, Liz Truss, the Chief Secretary to the Treasury and a close ally of Mr Johnson, insisted the successor to Sir Kim should not be chosen by Theresa May but her successor in Downing St.

No 10 has refused to be drawn on a timeline for the replacement; Mrs May’s spokesman, when pressed, would only say an announcement about Sir Kim’s successor would be announced “in due course”.

However, he did confirm that, technically, Donald Trump would have a veto over any new appointee.

He explained: “It is for the UK to select our ambassadors to all countries. As with all ambassadorial appointments, there is a process go through with the host country but that is invariably a formality.”

When it was pointed out that the appointment of an ambassador, therefore, did not have to be a formality, he replied: “It’s certainly very unusual for a nomination not to be accepted.”

Sir Kim resigned after the leak of sensitive diplomatic dispatches detailing his views on the "inept" Trump administration. He said the White House’s decision not to deal with him meant his role had become impossible. It was also suggested that Mr Johnson’s refusal to back the ambassador during the TV leadership debate with Jeremy Hunt was a “factor” in his decision to quit.

Asked whether Mrs May, who leaves office on July 24, should appoint Sir Kim’s successor, Ms Truss said: "The Permanent Secretary of the Foreign Office has been clear that this is a job that is going to take months rather weeks to recruit for, so the question is academic.

"It's likely that the new ambassador will be selected and appointed by the new prime minister," she added.

Ms Truss also said she did not like foreign leaders "slagging off" the UK after Mr Trump's Twitter tirade about Mrs May's handling of Brexit but insisted the PM’s deal was a “dead duck” and a fresh approach was needed to get Britain to leave the EU by October 31.

Meanwhile, Mr Hunt has sent a message of reassurance to Foreign Office staff, saying: "Please keep speaking up without fear or favour, remembering that the UK Government alone will determine appointments based on our national interest. As Foreign Secretary and a proud leader of the world’s greatest diplomatic service, I want you to know you will always get all the support you need to carry out your vital work. I will ensure you get it."

In Washington, foreign ambassadors have let it be known they shared Sir Kim’s view of the "dysfunctional" Trump White House with one saying about the leaked cables: "It could have been any of us."