Scotland Yard has launched a criminal investigation into the leak of diplomatic dispatches sent by Britain’s US ambassador Sir Kim Darroch.
The inquiry will be carried out by the Metropolitan Police counter terrorism command which is responsible for investigating breaches of the Official Secrets Act.
Assistant commissioner Neil Basu said: “Given the widely reported consequences of that leak I am satisfied that there has been damage caused to UK international relations, and there would be clear public interest in bringing the person or people responsible to justice.
“The investigation will be reviewed at every stage to ensure a proportionate investigation is undertaken.”
Mr Basu also issued a warning to the media that any further publication of the ambassador’s cables could be a criminal offence.
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”.
His 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.
Mr Basu said the Met had been brought in following a cross-government investigation led by the Cabinet Office into the leak of the cables to The Mail on Sunday.
In a statement, he urged whoever was responsible to turn themselves in and face the consequences of their actions.
“I would say to the person or people who did this, the impact of what you have done is obvious. However, you are now also responsible for diverting busy detectives from undertaking their core mission.
“You can stop this now. Turn yourself in at the earliest opportunity, explain yourself and face the consequences.”
In an unusual move, Mr Basu also warned owners, editors and publishers of social and mainstream media not to publish leaked government documents, and to return any in their possession to their rightful owners.
“The publication of leaked communications, knowing the damage they have caused or are likely to cause may also be a criminal matter,” he said.
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”.
Meanwhile, Mr Trump has suggested he was too quick to denounce Sir Kim following the leak.
Speaking to reporters at the White House, Mr Trump said he had since been told Sir Kim had said “some very good things” about him.
“I wish the British ambassador well. Some people just told me – too bad – they said he actually said some very good things about me,” he said.
“I wish the British ambassador well but they have got to stop their leaking problems there just like they have to stop them in our country.”