The Metropolitan Police has launched a criminal investigation into the alleged leak of diplomatic emails from the UK ambassador in the US, which were critical of the Trump administration.
Assistant Commissioner Neil Basu said there was a “clear public interest” in bringing those responsible to justice.
Sir Kim Darroch stepped down as ambassador on Wednesday, saying it was “impossible” for him to continue.
President Trump had earlier said the US would no longer deal with Sir Kim.
The US president branded him “a very stupid guy” after confidential emails emerged where the ambassador had called his administration “clumsy and inept”.
Announcing the criminal investigation, Mr Basu said he was satisfied the alleged leak had damaged UK international relations.
He urged whoever was responsible to turn themselves in and “face the consequences”.
“I would say to the person or people who did this, the impact of what you have done is obvious,” he said.
“However, you are now also responsible for diverting busy detectives from undertaking their core mission.”