Scotland Yard’s warning to the media about publishing more leaked diplomatic cables from Britain’s ambassador to the US has been deemed unacceptable and akin to the approach of a “totalitarian” regime.

The Society of Editors and former chancellor George Osborne have both criticised the Metropolitan Police for encroaching on press freedom after launching a criminal investigation into the leak of diplomatic dispatches sent by Sir Kim Darroch.

The inquiry by the Met’s counter terrorism command, which is responsible for investigating breaches of the Official Secrets Act, was announced in a statement attributed to Assistant Commissioner Neil Basu.

Executive director of the Society of Editors Ian Murray said: “I cannot think of a worse example of a heavy-handed approach by the police to attempt to curtail the role of the media as a defence against the powerful and those in authority.

“The implied threat is that the media would be acting against the law in publishing leaked documents, even if they were in the public interest.

“This is simply not acceptable in a free society and will act as a huge deterrent to whistle-blowers.

“Frankly it is the kind of approach we would expect from totalitarian regimes where the media are expected to be little more than a tame arm of the government.

“This is not nor should not be the case here in the UK.

“It is ironic indeed that Scotland Yard’s approach comes in the week where the UK has hosted the first Global Conference for Media Freedom.

“And we should not forget that the UK already languishes at number 33 in the World Index on Press Freedom created by Reporters Without Borders.

“To be a true beacon to the world on press freedom, the media in the UK should not have to face threats from the police in this way.”

Meanwhile, Mr Osborne, now editor of the Evening Standard, appeared to suggest the statement which called for any leaked documents to be returned to the Government was written by a junior officer and Mr Basu should distance himself from the comments.

The statement from Mr Basu said: “The publication of leaked communications, knowing the damage they have caused or are likely to cause may also be a criminal matter.

“I would advise all owners, editors and publishers of social and mainstream media not to publish leaked government documents that may already be in their possession, or which may be offered to them, and to turn them over to the police or give them back to their rightful owner, Her Majesty’s Government.”

Mr Osborne described the statement as “very stupid and ill-advised”.

“If I were the Metropolitan Police Commissioner, and I wanted to maintain my credibility and the credibility of my force, I would quickly distance myself from this very stupid and ill-advised statement from a junior officer who doesn’t appear to understand much about press freedom,” the former chancellor tweeted.

Sir Kim announced on Wednesday he was resigning, saying his position had become “impossible” following the leak of his dispatches in which he described Donald Trump’s White House as “inept” and “dysfunctional”.

Those comments drew a furious response from the president who denounced him as a “very stupid guy” and a “pompous fool” and said the the White House would no longer deal with him.

In the Commons on Thursday, Foreign Office minister Sir Alan Duncan said an internal Whitehall inquiry had found no evidence the leak was the result of computer hacking.

Instead he told MPs the focus was on finding “someone within the system who has released illicitly these communications”.