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”.