Memos from Darroch describing the Trump administration as inept were leaked to a British newspaper, infuriating the US president, who launched a Twitter attack on both the envoy and outgoing prime minister Theresa May, who had given Darroch her full support.
"We are, however, a body charged with enforcing the law, and we have received legal advice that has caused us to start a criminal inquiry into the leak of these specific documents as a potential breach of the Official Secrets Act (OSA)".
In a statement, the London Metropolitan police's assistant commissioner, Neil Basu, said that "given the widely reported consequences of that leak", there had been "damage caused to United Kingdom global relations" and that there "would be clear public interest in bringing the person or people responsible to justice".
Basu addressed the person responsible for the leak directly, saying: "The impact of what you have done is obvious". You can stop this now.More news: Tour de France rider’s $12k bike SPLITS IN HALF during crash
At the same time, Assistant Commissioner Neil Basu urged journalists in possession of leaked Government documents to return them, warning any further publication from the dispatches cables could result in prosecution.
The head of Britain's diplomatic service, Simon McDonald, said this week he was "bracing" himself for further leaks.
Darroch's defenders said his critical memos showed he was doing his job by providing candid assessments, as diplomats are expected to do, but he said the controversy had made it impossible to fulfill his duties.
"I fear there may be more", he said.
"(Darroch) said that what somebody had relayed to him had certainly been a factor in his resignation", he told BBC television in a testy interview.
Sir Kim quit his post amid diplomatic furore, after memos he wrote which criticised the Trump administration were published.More news: NY power outage knocks out subways, businesses, elevators
"I can not think of a worse example of a heavy-handed approach by the police to attempt to curtail the role of the media as a defence against the powerful and those in authority", he said.
Boris Johnson (L) refused to back British ambassador Kim Darroch (R) during a televised debate.
At one point he also goaded Hunt, asking how long he would keep Darroch in place after vowing to retain him.
Asked in Friday's interview if he would be "as craven" a prime minister as he has been a candidate, Johnson said: "We have been very forthright with the United States of America and I will continue to be forthright".
The new prime minister will be named on July 23 and take office the following day.More news: Jets TE Herndon suspended for four games