This is a threat from a senior police officer in the UK to journalists. The context is the leak of diplomatic cables which showed that the UK Ambassador to the USA has a low opinion of Trump and his presidency. But it could be any other leak of information from the government to the press.
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.
The law in question is probably the Official Secrets Act. However; the provisions of this Act apply to government officials only and not to journalists. The Met’s Assistant Commissioner Neil Basu who made these comments appears to be attempting to extend the provisions of the Official Secrets Act to the media. Arbitrarily and without an Act of Parliament.
It is an easy game to play. But, let’s play it anyway. Imagine if this was Russia – a senior FSB official threatened journalists en masse with criminal prosecution for publishing leaked government information….
It isn’t just The New Observer who finds this latest sign of rampant power a little shocking. This, for example, is George Osborne, former Minister of State, and now a newspaper editor:
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. 
However, The Guardian, chooses to lead with the comments of another former Minister, the ex Defence Secretary, Michael Fallon, in support of the police staetment. The Guardian reports Fallon as saying the remarks were “quite logical”. 
However; despite The Guardian’s headline “Fallon says press should be subject to Official Secrets Act” a full perusal of his comments as reported in the paper actually falls short of that. The Guardian writes that in response to a question whether journalists should be subject to the Official Secrets Act Fallon replied:
I don’t think anyone can entirely absolve themselves of the need to avoid damage to this country.
We have press freedom … but we also have laws. We have the Official Secrets Act and it is important that law is upheld
Which means that Fallon is siding with the police while being careful to show some awareness that the Official Secrets Act does not currently apply to the Press.
Update – 13/7/19
The Met Police have issued a clarification. This is presented by The Guardian as a “rowback”. But it isn’t:
However, we have also been told the publication of these specific documents, now knowing they may be a breach of the OSA, could also constitute a criminal offence and one that carries no public interest defence
“We have been told”? Interesting use of the passive to conceal the source of this advice. The Attorney General? The CPS? Most likely the former since this witch hunt seems to be driven from the top. Nor is the specific criminal offence mentioned.
The New Observer is not in a position to evaluate the legal claim here but – it appears that the Official Secrets Act only applies to people working for the state – not journalists. Also; notice the key phrase here “now knowing they may be a breach of the OSA” and consider this together with the earlier statement about how the leak has caused “damage” to Britain’s international relations. What law exactly does Neil Basu have in mind? Is his advice (wherever it comes from) that journalists could be prosecuted for treason for publishing these materials?