United States President Donald Trump's decision to pull out of a landmark nuclear deal with Iran was done so to spite his predecessor, Barack Obama, according to further leaked memos from Britain's outgoing American ambassador, Sir Kim Darroch.

Key points: Leaked diplomatic cables accuse Donald Trump of "diplomatic vandalism"

Leaked diplomatic cables accuse Donald Trump of "diplomatic vandalism" Mr Trump pulled the US out of a landmark nuclear deal with Iran in 2018

Mr Trump pulled the US out of a landmark nuclear deal with Iran in 2018 The deal eased economic sanctions on Tehran in exchange for curbs on its nuclear program

British newspaper The Mail on Sunday published memos from 2018 where Sir Kim labelled Mr Trump's decision to pull out of the Iran nuclear deal an act of "diplomatic vandalism".

The memo was written after then-foreign secretary Boris Johnson visited Washington in a failed attempt to persuade the US not to abandon the Iran nuclear agreement.

The agreement — signed in 2015 between Iran, US, Britain, France, China, Russia and Germany — eased international sanctions on Iran in exchange for Tehran temporarily halting its nuclear program.

Historically, Iran's neighbours — including Israel and Saudi Arabia — have feared Iran's nuclear program, and have argued against further concessions to Tehran.

In 2018, Mr Trump pulled the US out of the deal and imposed tough sanctions on the country.

The European Union and Russia remain committed to the deal.

"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 Obama's deal," Sir Kim wrote.

Boris Johnson's soft line on Trump had consequences: Sir Kim

Sir Kim labelled the Trump administration dysfunctional and inept. ( AP: Sait Serkan Gurbuz )

Sir Kim's frank admission has been revealed from more leaked memos from the Mail on Sunday newspaper, despite warnings from police that doing so might be a crime.

The ambassador resigned last week after the newspaper published cables in which he had branded the Trump administration dysfunctional and inept.

He remains formally in the post while a successor is chosen for one of Britain's most important diplomatic jobs.

The White House made his situation untenable after it refused to engage with the diplomat after Mr Trump branded the ambassador a "pompous fool" in a Twitter tirade.

British politicians and officials have expressed dismay about the fact that the British ambassador was forced to step down because of pressure from a foreign leader.

Some also blame Mr Johnson, who is likely to become Britain's next prime minister, for refusing to publicly defend the ambassador after Mr Trump posted disparaging tweets about the ambassador for two days.

Sir Kim has said Mr Johnson's silence contributed to his decision to quit.

Press freedom defended as Brexit conspiracy rumours swirl

The leaks against Sir Kim have been seen to benefit Brexiteers such as Nigel Farage, left. ( AP: Gerald Herbert )

British police are hunting the culprits behind the leak — and, contentiously, have warned journalists that publishing the documents "could also constitute a criminal offence".

Citizens across the United Kingdom have a legislated right to freedom of expression under the Human Rights Act of 1998.

Rumours are swirling in government circles in London about who was behind the leak, widely seen as benefiting supporters of Brexit and Trump.

The journalist who reported the cables, Isabel Oakeshott, is a strong supporter of Brexit and an ally of Brexit Party leader Nigel Farage, who also is Britain's leading champion of Mr Trump.

Loading

Mr Farage has accused Sir Kim of lacking enthusiasm for Britain's departure from the EU and said he should be replaced with "a non-Remainer who wants a trade deal with America."

Mr Johnson and Jeremy Hunt, the two contenders to become Britain's next prime minister, have defended the media's right to publish.

"We have to make sure that we defend the right of journalists to publish leaks when they are in the national interest," Mr Hunt said.

This was echoed by a statement from the Mail on Sunday:

"Our readers across the globe now have important information about how Britain tried, but failed, to stop President Trump abandoning the Iran nuclear deal," the newspaper said in a statement.

"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?"

British officials have said they have no evidence that hacking was involved in the documents' release, and that the culprit is likely to be found among politicians or civil servants in London.

Police are investigating the leak as a potential breach of the Official Secrets Act, which bars public servants from making "damaging" disclosures of classified material.

Breaking the act carries a maximum sentence of two years in prison, though prosecutions are rare.

ABC/AP