Scotland Yard said it respected media rights, but that the recent release of leaked diplomatic notes was not in the public interest.
The police initiated a criminal investigation into the flight of diplomatic emails from the British ambassador to the United States, Sir Kim Darroch.
Neil Basu, deputy commissioner of the metropolitan police, said that publishing e-mails could be a crime.
Publishers have criticized a previous warning statement against further publications.
Mr. Basu said the police have "no intention to prevent publishers from publishing stories in the public interest of a liberal democracy".
However, he said that the metropolitan police were told that the publication of "these specific documents … could also constitute a criminal offense and one that does not bring any public interest defense".
"We know that these documents, and potentially others, remain in circulation," he added.
The government has already opened an internal investigation into the publication of memos, which were critical of the Trump administration – and sparked a furious reaction from the US president, who said he would no longer have to do with Sir Kim.
The line started a week ago, when confidential e-mails emerged in which the British ambassador to the United States called Donald Trump's administration "clumsy and inept".
President Trump has branded Sir Kim "a very stupid boy" and Sir Kim resigned on Wednesday, saying it was "impossible" for him to continue.
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A a criminal investigation into the loss was initiated by the Met Terrorism Control Command, which assumes national responsibility for investigating allegations of criminal violations of the official secrets law.
Mr. Basu said he was satisfied with the flaw that had damaged the UK's international relations and added that there was a "clear public interest" in bringing the perpetrators to justice.
However, he faced a negative reaction after advising people and the media not to publish widespread government documents and hand them over to the police or return them to the rightful owner.
The editor of Evening Standard George Osborne described the Met's statement as "stupid" and "reckless".
The Sunday Times political editor Tim Shipman has labeled it "sinister" and "anti-democratic". "Do you have any understanding of a free society? This is not Russia," he tweeted.
Mr. Basu issued a further statement Saturday afternoon stating that he had received a legal opinion that led the Met to initiate a document investigation as a potential violation of the Official Secrets Act (OSA).
"We have a duty to prevent and detect the crime and the previous statement was intended to alert the risk of breach of the OSA," he said.
What is "in the public interest"?
Journalists are not above the law, but it is intended "in a free, liberal and democratic society that the media" should be free to report on leaked documents that they believe are of public interest, "says Ian Murray, executive director of the Society of Publishers.
Murray argues that the police are unlikely to "shoot the messenger", adding that it is necessary to authorize journalists not to be bullied in delivering the documents.
Who decides what is in the public interest, however, can be controversial.
"It's a difficult line to cover between what is in the public interest and what the public is interested in," says Murray.
However, he adds that the idea that a particular body would make that decision – or that no one should decide because journalists have "docilely" delivered documents to the police is "terrifying".