The Metropolitan Police has insisted it "respects the rights of the media" but that it had been advised publication of leaked emails from Sir Kim Darroch could be a criminal offence.
It comes after a huge row in which the force was accused of trying to restrict press freedom when Assistant Commissioner Neil Basu "advised" editors not to publish leaked documents.
Tory leadership candidates Boris Johnson and Jeremy Hunt had led the criticism of Mr Basu's remarks on Saturday, with frontrunner Mr Johnson saying a prosecution for publishing leaked documents would have a "chilling effect on public debate".
But the force hit back, saying it was given legal advice that the publication of diplomatic cables relating to ex-ambassador Sir Kim may be a breach of the Official Secrets Act (OSA) with "no public interest defence".
Mr Basu said in a statement: "The Metropolitan Police respect the rights of the media and has no intention of seeking to prevent editors from publishing stories in the public interest in a liberal democracy. The media hold an important role in scrutinising the actions of the state.
“We are however a body charged with enforcing the law, and we have received legal advice that has caused us to start a criminal enquiry into the leak of these specific documents as a potential breach of the OSA. The focus of the investigation is clearly on identifying who was responsible for the leak.
“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 know these documents and potentially others remain in circulation.
“We have a duty to prevent as well as detect crime and the previous statement was intended to alert to the risk of breaching the OSA.”
Sir Kim, Britain's ambassador to Washington, quit on Wednesday after the leak of diplomatic cables he had written which were damning about Donald Trump’s administration.
Sir Kim said the storm over the unauthorised disclosures had made it “impossible for me to carry out my role as I would like”.
The leak sparked an extraordinary trans-Atlantic row with Mr Trump launching Twitter attacks on Sir Kim and escalating them into criticism of Theresa May.
