The suspect behind the leak of confidential memos from Britain’s ambassador to the US has been identified, it has been reported, as fresh details of their content was published.

Sir Kim Darroch resigned last week after the Mail on Sunday published extracts from the memos, in which he described Donald Trump’s administration as “clumsy and inept”.

The Metropolitan police’s counter-terrorism command launched a criminal investigation on Friday, and on Sunday, the Sunday Times, citing unnamed government sources, reported that a suspect had been identified and the possibility of a computer hack by a foreign state ruled out.

“They think they know who did the leaking,” the paper quoted an unnamed government source as saying. “It’s now a case of building a case that will stand up in court. It was someone with access to historical files. They went in and grabbed a range of material. It was quite crude.”

Kim Darroch saga ‘is part of assault on civil service’ Read more

Both the Sunday Times and the Mail on Sunday reported that intelligence officials from GCHQ were about to join the investigation, to attempt to find the suspect by examining email and phone records.

Announcing the police investigation on Friday, Met assistant commissioner Neil Basu warned media organisations that they could be breaking the law if they published further details from the cables.

However, the Mail on Sunday published more details on Sunday, quoting Darroch accusing Donald Trump of abandoning the nuclear deal with Iran out of spite towards his predecessor Barack Obama.

In a telegram to the then foreign secretary, Boris Johnson, who has been blamed for the ambassador’s resignation because of his failure to back Darroch after the initial leak, Darroch wrote: “On the substance, the [US] administration is set upon an act of diplomatic vandalism, seemingly for ideological and personality reasons – it was Obama’s deal.”

On Saturday, the Met rowed back from its threat that journalists could face prosecution if they published further leaked diplomatic cables after widespread criticism – including from Johnson, and his rival for the Conservative leadership, Jeremy Hunt – that the force was attacking press freedom. Basu clarified that the Met “respects the rights of the media and has no intention of seeking to prevent editors from publishing stories in the public interest in a liberal democracy”.

A spokesman for the Mail on Sunday said it was continuing to publish details from the leaks because it contained “important information about how Britain tried, but failed, to stop President Trump abandoning the Iran nuclear deal. What could be more in the public interest than a better understanding of how this position was reached, which may have serious consequences for world peace?”

A Foreign Office spokesman said: “A police inquiry into the totally unacceptable leak of this sensitive material has begun. The perpetrator should face the consequences of their actions.

“It’s not news that the US and UK differ in how to ensure Iran is never able to acquire a nuclear weapon; but this does underline that we do not shy away from talking about our differences and working together.”