At least all our leaders defend press freedom, but they came forward too late for Kim Darroch

The easy part is the defence of the right of a free press to publish. It was, as George Osborne, editor of the Evening Standard, said, “ill-advised” of Neil Basu, assistant commissioner of the Metropolitan Police, to threaten journalists with prosecution for publishing leaked documents.

Boris Johnson and Jeremy Hunt, one of whom will become prime minister this month, both condemned the statement. Mr Johnson said: “A prosecution on this basis would amount to an infringement on press freedom and have a chilling effect on public debate.”

The consensus that Mr Basu had made a mistake extended to Jeremy Corbyn, who might become prime minister on a longer timescale, and who said it was right there were “considerable protections” for journalists who reveal information.

In fact, the protections of free speech in Britain are not as well established as Mr Corbyn implied. The courts have developed a public interest defence through case law, and juries have been reluctant to send leakers to jail – Clive Ponting, for example, being found not guilty in 1985, although he admitted leaking secrets about the sinking of the Belgrano in the Falklands war.

The more difficult questions in the current case, however, are whether Mr Johnson should have withheld his support from Sir Kim Darroch, the UK ambassador to Washington, and whether Sir Kim should have resigned.

There is little doubt that Sir Kim was the victim of a politically motivated leak last weekend of some of his disparaging assessments of the Trump administration – which he had called “inept” and “dysfunctional”. It would seem that he was brought down by someone who wanted him replaced, under a Johnson government, by someone who was more sympathetic to Trump, and therefore to Brexit.

Mr Johnson was spineless and shortsighted to refuse to back Sir Kim in the ITV leaders’ debate on Tuesday. Instead, he tried to sound tough by saying that he, as prime minister, and no one else, would decide who our ambassador to the US should be. Mr Johnson’s equivocation was part of the reason Sir Kim resigned on Wednesday.

At yesterday’s hustings Mr Johnson finally admitted he had made a mistake: “I probably should have been more emphatic that Kim personally had my full support.”

Of course, the Mail on Sunday was right to publish Sir Kim’s disobliging cables.

We do not agree that the public interest was served by the ensuing damaging of US-UK relations, or by the discouragement of our diplomats from expressing themselves candidly in writing.

But the test for disclosure is not our subjective view of the public interest; it is the broader view of when disclosure must be assumed to be in the public interest – which it must be, unless there are compelling reasons of national security or individual safety which preclude publication.

Mr Johnson, as he now admits, should have tried to keep Sir Kim in post. He will need the loyalty of our diplomatic staff if he becomes prime minister. But if there is one silver lining to this affair, it is that all our leading politicians have been united in defending the freedom of the press.