Standing in front of Corinthian columns at the US Department of State, Boris Johnson vigorously pumped the hand of his American counterpart Mike Pompeo and gave a thumbs-up to reporters.

But as the Foreign Secretary turned on his heels after the photocall, he knew that his hastily arranged diplomatic mission to Washington was on the brink of failure.

It was May 7, 2018, and Mr Johnson had been hurriedly dispatched to make a last-ditch plea to President Donald Trump not to abandon a nuclear deal with Iran, regarded by many experts as critical to prevent the regime from building an atomic bomb.

During a frantic 26 hours of meetings, Mr Johnson met all the key 'Trump Whisperers' – those advisers with the ear of America's unpredictable leader. It became clear, however, that despite the 'special relationship' between Britain and the US, Mr Johnson was not going to change the President's mind.

Sir Kim said in his cables that the president abandoned the deal for 'personality reasons' and suggested there were splits among his closest advisors

Britain’s Ambassador to Washington Sir Kim Darroch claimed that Donald Trump abandoned the Iran nuclear deal as an act of ‘diplomatic vandalism’ to spite his predecessor Barack Obama

In an explosive diplomatic cable sent to London hours after Mr Johnson returned home, Sir Kim Darroch, Britain's Ambassador to the US, lamented that Mr Trump's Administration 'is set upon an act of diplomatic vandalism'.

In a blunt assessment, Sir Kim wrote that Mr Trump appeared to be abandoning the Iran nuclear deal for 'personality reasons' –because it had been agreed by his predecessor Barack Obama.

And Sir Kim suggested there were splits among the President's closest advisers and claimed the White House lacked a 'day-after' strategy on what to do after withdrawing from the Joint Comprehensive Plan of Action (JCPOA), the official name for the Iran deal.

The new telegram is disclosed today, a week after this newspaper first revealed an astonishing cache of leaked memos from Sir Kim.

Our revelations, which have dominated headlines on both sides of the Atlantic, infuriated the President and prompted him to brand Sir Kim a 'pompous fool' and 'wacky' and declare on Twitter that he would no longer deal with him.

During a frantic 26 hours of meetings, Boris Johnson met all the key 'Trump Whisperers' but despite vigorously shaking hands with US Secretary Mike Pompeo and giving a thumbs-up to cameras, he was unable to change the US president's mind

Sir Kim resigned on Wednesday after Mr Johnson refused to say during a televised Tory leadership debate whether he would keep the Ambassador in his job if he became Prime Minister. Appearing alongside leadership rival Jeremy Hunt, Mr Johnson expressed only mild criticism of Trump's comments and refused to back the highly experienced diplomat.

His cold shouldering of Sir Kim was in stark contrast to their joint efforts 14 months earlier to try to convince the White House not to junk the Iran nuclear deal.

Signed in July 2015 by Iran and six major powers, including the US and Britain, the deal limited Iran's nuclear programme in return for the lifting of crippling sanctions.

Under the accord, Iran promised to slash its uranium stockpile by 98 per cent for 15 years, dramatically reduce its number of centrifuges which can be used to enrich uranium, and allow forensic inspections of its nuclear facilities.

In a blunt assessment, Sir Kim wrote that Mr Trump appeared to be abandoning the Iran nuclear deal for 'personality reasons'

According to the Obama Administration, which viewed it as a major diplomatic triumph, the deal would increase the so-called 'break-out time' – the time it would take Iran to produce enough highly enriched uranium to build a nuclear bomb – from a couple of months to a year or more.

But Mr Trump hated the pact, which he branded a 'disaster' and 'the worst deal ever negotiated' during his 2016 election campaign.

To the dismay of his future allies in Europe, he declared before the election that his 'number one priority' would be to 'dismantle the disastrous deal with Iran'. It was perhaps, therefore, unsurprising that Mr Johnson was in an uncharacteristically sober mood as he settled into his eight-hour flight to Washington DC almost two years later on May 6, 2018.

Sir Kim had done a good job of organising what he would later describe in his diplomatic telegram as 'a packed programme' for the Foreign Secretary.

Boris Johnson did not meet the President himself but did become the fist minister to meet Vice President Mike Pence. Sir Kim says it was clear the president's advisers had differing opinions

During his short visit, Mr Johnson would see Mr Trump's National Security Adviser, John Bolton, and his Chief of Staff John Kelly, as well as Trump's daughter Ivanka, and her husband Jared Kushner, who was heavily involved in the Middle East peace process.

By a stroke of luck, there would also be an opportunity to meet Mr Pompeo, Mr Trump's newly appointed Secretary of State, whose planned trip to North Korea had been delayed.

But disappointingly, Mr Johnson would not be seeing the President himself. Everything had happened too fast and he would have to make do with meeting Vice President Mike Pence, even if this would make him the first UK Minister to secure face-time with Trump's deputy.

In a canny move, however, Johnson appeared on Fox and Friends – Mr Trump's favoured TV news show – and made an impassioned and direct appeal for the President to change course.

'What if the Iranians do rush for a nuclear weapon?,' he asked. 'Are we seriously saying that we're going to bomb those facilities? Is that really a realistic possibility or do we work with what we've got and push back on Iran together?'

Boris Johnson, pictured with Ivanka Trump, travelled to the US in 2018 in a doomed trip to persuade Donald Trump not to abandon the Iran deal

In his memo, Sir Kim details how, during his crunch meetings with Trump's inner circle, Mr Johnson lavished praise on the Americans for their work in North Korea; thanked them for their support over Russia's poisoning of Sergei and Yulia Skripal in Salisbury; and acknowledged the 'impeccable cooperation' between the US and the UK over recent air strikes in Syria.

Such flattery came easily to the Old Etonian. The main subject on the agenda – Iran – would be much more tricky.

In the two-page summary, written by Sir Kim to Mr Johnson and sent to the Foreign Office, No 10, the Ministry of Defence and elsewhere in Whitehall, the Ambassador revealed how the Foreign Secretary said he 'recognised the flaws and inadequacies in the deal' but argued the White House should stick with it. Paraphrasing Mr Johnson's argument, he wrote: 'A deal which established a break-out period of more than a year, took away two-thirds of Iran's centrifuges, reduced their enriched uranium stockpiles by 95 per cent, concreted over the heavy water reactor [potentially used to produce plutonium that could be used in a bomb] and included an exceptionally rigorous inspection regime, should not be thrown away.'

Vice President Pence, Mr Bolton and Mr Pompeo listened respectfully – with the latter showing signs that he disagreed with Mr Trump's position, according to the cable.

Mr Trump hated the pact negotiated by his predecessor, which he branded a 'disaster' and 'the worst deal ever negotiated' during his 2016 election campaign

During their meeting on the seventh floor of the Department of State, Mr Pompeo 'did some subtle distancing by talking throughout about 'the President's decision' ' and hinted that he had tried – and failed – to 'sell' a revised text to Trump, according to Sir Kim's memo.

He also hinted to Mr Johnson that he was hoping to persuade the President to soften the impact of axing the deal by, for example, holding off imposing so-called secondary sanctions designed to punish other countries for doing business with Iran. After their talks, Mr Johnson and Mr Pompeo briefly appeared before reporters in the Department's Treaty Room Suite, shaking hands and exchanging stilted pleasantries.

Sir Kim added in his memo to Johnson: 'Pence also took you aside and hinted that the President was looking for a new deal which 'covered everything': stopping Iranian nuclear activity – enrichment, production, reprocessing – entirely; halting their ballistic missile development; and forcing them to back off from their activities in the region.'

'Absolutely wonderful to see you,' gushed the Foreign Secretary. 'Thank you for making the time.'

'These are important times, so thank you very much for being here,' replied Mr Pompeo, who did not answer a reporter who asked whether he had been 'persuaded' by Mr Johnson.

But Vice President Pence had a blunter message during his meeting with the Foreign Secretary in the White House.

Sir Kim's report details how he told Mr Johnson that Mr Trump's 'clear inclination' was 'to start again rather than build on faulty foundations: he wanted a much tougher deal.'

Sir Kim added in his memo to Johnson: 'Pence also took you aside and hinted that the President was looking for a new deal which 'covered everything': stopping Iranian nuclear activity – enrichment, production, reprocessing – entirely; halting their ballistic missile development; and forcing them to back off from their activities in the region.'

In the interests of diplomacy, Mr Pence reassured Mr Johnson that the White House would want to stay close to its allies and consult the British about next steps.

'This is not about walking away, it's about walking to something better,' the Vice President reportedly said.

However, Sir Kim described how the British delegation was surprised by the inability of any of Mr Trump's advisers to explain why he was so determined to junk the deal or their strategy after withdrawing from it.

Darroch said Mr Trump's decision to withdraw from the deal was 'the end of a long road' and highlighted how National Security Adviser John Bolton's arrival in the White House in April 2018 'was widely predicted to cement this outcome'

'None of the three could articulate why the President was determined to withdraw, beyond his campaign promises,' he wrote. 'And, even when you pressed, none had anything much to say about the day after, or a Plan B, beyond reimposition of US sanctions.'

Such damning criticism echoed another memo, revealed by this newspaper last week, in which Sir Kim described the White House as a 'uniquely dysfunctional environment' and 'diplomatically clumsy and inept'.

The one crumb of comfort for Mr Johnson was that the ultra-hawkish Mr Bolton, who has previously called for the bombing of the Iranian regime, 'said explicitly that he [Trump] wasn't favouring a military option'.

Who is the officer threatening press with prosecution over the Sir Kim Darroch leak? Assistant Commissioner Neil Basu He is the Scotland Yard high-flyer with what many regard as the toughest job in policing. Assistant Commissioner Neil Basu, Britain’s top Asian police officer, oversees terrorism investigations at the Metropolitan Police and is the so-called ‘national lead’ officer for counter-terror operations across the UK. Colleagues say he is well-liked within the force and by intelligence officials at MI5 and is likely to be a contender to be the next Met Commissioner. Yet his 27-year police career has not been without controversy, most notably as head of Operations Weeting, Elveden and Tuleta. The three inquiries into phone hacking, computer hacking and alleged payments to police officers by newspapers cost around £19.5 million and were criticised for criminalising journalists. Critics at the time said the Met could have spent the money going after terrorists, murderers and drug dealers. Mr Basu also raised eyebrows when he criticised the Prevent programme – which tries to detect and deradicalise Muslim extremists – as ‘toxic’. ‘Government will not thank me for saying this, but an independent reviewer of Prevent... would be a healthy thing,’ he said. A Hindu, born to an Indian doctor father and a white British mother, he has said he has encountered racism over most of his life. He grew up in Stafford, where he studied at Walton High School before reading economics at Nottingham University. He became a Met police officer in 1992, serving first as a beat bobby in Battersea, South London, then swiftly moving through the ranks as a borough commander in Barnet, North London, and a Commander of South London in 2012. His first major high-profile Met post came in 2014, when he was appointed Commander – Organised Crime and Gangs. Three years later, as a Deputy Assistant Commissioner, Mr Basu was tested as Britain was hit by an unprecedented five terrorist attacks in one year, including the Manchester bombing that killed 22 people and the Westminster attack, which killed four, including a police officer. The most-high profile counter-terrorism investigation overseen by Mr Basu in his current role was the poisoning of Sergei and Yulia Skripal in Salisbury last year, which the Met says was directed by the Kremlin. A father with three sons, Mr Basu is married to Dr Nina Cope, a senior official at the National Crime Agency, often described as Britain’s FBI. Advertisement

At the end of a gruelling series of meetings, the Foreign Secretary left Washington empty-handed.

Sir Kim could not hide his disappointment when at 1.38pm in Washington on May 8, he sent his cable.

'I'm grateful to you for coming out on short notice and undertaking such a packed programme,' he 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 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.'

Less than two hours later, the failure of Mr Johnson's mission was confirmed when Mr Trump announced he was terminating America's participation in the Iran deal and re-imposing sanctions.

In a second cable sent at 6.58pm Washington time, Sir Kim reported that 'following a typically hyperbolic statement on the nature of the 'murderous' Iranian regime, Mr Trump signed a presidential memorandum to start the process of reinstating US nuclear sanctions.'

Mr Trump's decision was, he said, 'the end of a long road' and highlighted how Mr Bolton's arrival in the White House in April 2018 'was widely predicted to cement this outcome.'

'Much of the speech echoed lines we have heard from Bolton in person,' he explained.

Before signing off for the night he reassured London that British diplomats had pulled out all the stops. It was clear, however, that the special relationship had its limits.

'We did everything we could across government to save the deal. Many said it would go on the first day of Trump's time in office. There has been no stone unturned in our attempt to persuade and shape the Administration's view,' he sighed.

The same frustration was evident the following day when Mr Johnson made a statement to the House of Commons about the President's decision.

'The Government regret the decision of the United States Administration to withdraw from the deal and reimpose American sanctions on Iran,' he told MPs.

'We did our utmost to prevent this outcome: from the moment that President Trump's Administration took office, we made the case for keeping the JCPOA at every level.'

Choosing his words carefully, he also appeared to acknowledge Sir Kim's concerns at the White House's lack of a 'day-after' strategy. 'Now that our efforts on this side of the Atlantic have not succeeded, it falls to the US Administration to spell out their view of the way ahead.'

For Mr Johnson, however, the visit had not been completely in vain.

Two months later and despite his criticism of Mr Trump's withdrawal from the nuclear deal, the President used a visit to the UK to hailed the Foreign Secretary as 'a very talented guy' who 'would be a great Prime Minister'.

Now, with critics accusing him of throwing Sir Kim 'under the bus' to protect his leadership ambitions, Mr Johnson may have cause to regret receiving such a fulsome endorsement.