Britain's outgoing ambassador to the US said President Donald Trump abandoned the Iran nuclear deal in an "act of diplomatic vandalism", because it was agreed by his predecessor Barack Obama.

That is according to the latest leaked cables involving Kim Darroch.

The May 2018 memo was said to have been written by Mr Darroch after a visit to the US by Boris Johnson, then UK foreign secretary, who was making a final attempt to keep America in the deal.

Mr Darroch told Mr Johnson: "The outcome illustrated the paradox of this White House: you got exceptional access, seeing everyone short of the president; but on the substance, the administration is set upon an act of diplomatic vandalism, seemingly for ideological and personality reasons - it was [previous US president] Obama's deal.

"Moreover, 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."

Outgoing British ambassador to the US Kim Darroch hosts an event at the British embassy in Washington in 2017 | Image: Sait Serkan Gurbuz/AP/Press Association Images

The Iran accord, which is still backed by Britain, France and Germany, eased sanctions in return for cuts to Iran's nuclear programme.

It is the second set of leaked e-mails published by the Irish Mail On Sunday - last weekend they revealed that Mr Darroch had described Mr Trump's administration as "inept" and "dysfunctional", a revelation that eventually led to the ambassador's resignation.

British police are investigating the source of the leaks.

They warned that publishing further details of the ambassador's communications could be in breach of the UK's Official Secrets Act.

Responding in light of its latest story on the leaked cables, the Mail On Sunday said: "The media must be free to publish such information, in the public interest, as long as it does not endanger lives or national security.

A spokesman said: "Our readers across the globe now have 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?"

In response to the latest leak, a British Foreign Office spokesman said that the person responsible for the leak should "face the consequences of their actions".