Under the terms of the agreement - still supported by Britain, France and Germany - worldwide sanctions on Iran were eased in return for Tehran accepting curbs on its nuclear programme.
"They can't articulate any "day-after" strategy; and contacts with State Department this morning suggest no sort of plan for reaching out to partners and allies, whether in Europe or the region", Darroch wrote.
The Conservative leadership contender admitted he should have been more supportive of Sir Kim Darroch, and acknowledged that his refusal to explicitly back the ambassador had been a factor in his decision to step down.
Britain's police on Friday said they had opened an investigation into a leak of confidential memos that led to the resignation of the British ambassador to Washington.
Per some Whitehall sources, Sir Kim determined to resign after Mr Johnson failed to fully toughen him all thru a TV debate on Tuesday night. They say they want editors to know publishing them may be against the law because there is a criminal investigation underway into whether the original leak violated the Officials Secret Act.
Following the backlash, Asst Commissioner Basu added that the Met had "no intention" of trying to prevent the publication of stories in the public interest.
Current Foreign Secretary Jeremy Hunt said the leaks had: "Damaged UK/US relations and cost a loyal ambassador his job, so the person responsible must be held fully to account. We have a duty to prevent as well as detect crime, and the previous statement was meant to alert to the risk of breaching the OSA", he said.
"A prosecution on this basis would amount to an infringement on press freedom and have a chilling effect on public debate", Johnson said at a campaign event Saturday.
"The media must be free to publish such information, in the public interest, as long as it does not endanger lives or national security", the spokesman said.
The resignation is a totally remarkable situation, he said, which has been brought about by, first of all, the leaking of this confidential memoranda to London, anti-appropriate comments on his host government; secondly, by Donald Trump's public objection to the ambassador and to what he said, which itself is remarkable.
On Friday, the American president said that Britain needed to stop their "leaking problems". The perpetrator should face the consequences of their actions. Unlike many other news organizations, we have not put up a paywall.