Even as Sir Philip Barton wrote the letter to King Charles formalising the appointment of Lord Mandelson as US ambassador, he knew that serious trouble lay ahead.
The missive was sent on December 18, 2024, to Sir Clive Alderton, the King's long–serving private secretary, and included the observation that the Foreign Secretary David Lammy 'agreed with the appointment'.
The same could not be said of Barton. A lifelong career civil servant, the then Permanent Under Secretary at the Foreign Office had mounted a valiant rearguard action against the combined forces of Downing Street and the Cabinet Office as they fought to install the twice–sacked Cabinet minister in the most sensitive and powerful posting in the diplomatic service.
On Wednesday, it emerged that he is to appear next week in front of the Foreign Affairs select committee – the very committee that gave his successor Sir Olly Robbins a platform to eviscerate the Prime Minister on Tuesday – and Downing Street will be terrified that Barton will produce a smoking gun of his own.
He is certain to embarrass the Cabinet Office for a start. Despite Peter Mandelson's less than spotless record in public life, the Cabinet Office in particular pressed for him to be spared the traditional security vetting on the basis that he was a three–times Cabinet minister, a peer of the realm and a member of the Privy Council.
Intriguingly, the most senior minister in the Cabinet Office at the time was Pat McFadden, the Chancellor of the Duchy of Lancaster. When Mandelson was given a peerage in 2008 to become Business Secretary in Gordon Brown's government, it was McFadden who was made his deputy and became Mandelson's all–important eyes and ears in the House of Commons.
Indeed, McFadden grew to be such a close friend that he was one of the few politicians invited to Mandelson's wedding to his long–term partner Reinaldo Avila da Silva in October 2023.
But Barton was having none of the Cabinet Office's brinkmanship. As a man who had served as private secretary to four prime ministers, including John Major and Tony Blair, the London School of Economics–educated civil servant was more than capable of standing up for himself.
Sir Philip Barton (pictured) is to appear next week in front of the Foreign Affairs select committee
Pat McFadden (pictured) grew to be such a close friend of Mandelson that he was one of the few politicians invited to Mandelson's wedding to his long–term partner Reinaldo Avila da Silva in October 2023
He was rightly wary of Mandelson's commercial involvement with Chinese and Russian companies through Global Counsel, the lobbying firm he launched after he left Government in 2010.
Barton also had concerns about his links with the convicted paedophile Jeffrey Epstein. As unbelievable as it might sound, Mandelson stayed in the late financier's New York mansion in 2009 while Epstein was in jail for sex–trafficking an under–age girl.
A senior Whitehall source tells me: 'Barton pushed back at the Cabinet Office as he thought serious vetting was essential and Mandelson should not be waved through just because he was a Privy Councillor.
The issue went back and forth. But he wouldn't back down and the politicians became impatient.'
Even after the vetting process got underway, the pressure to rubber–stamp Mandelson's appointment was unrelenting.
Morgan McSweeney, who was at the time Sir Keir Starmer's all–powerful Downing Street chief of staff, is alleged to have told Barton: 'Just f*****g approve it.' The claim was put to Sir Olly Robbins during his grilling at the hands of the foreign affairs select committee on Tuesday following his abrupt sacking over the vetting row. McSweeney has denied saying it.
Throughout the fraught process to approve Mandelson's appointment, Barton was fighting the corner of Dame Karen Pierce, the outgoing US ambassador.
Most observers agreed that Pierce – no relation – had done a sterling job under President Joe Biden's administration and that she had already forged good contacts with the incoming Trump team, something Barton considered to be of crucial importance.
Former British ambassador to the U.S. Peter Mandelson walks with his dog outside his residence
Sir Olly Robbins (right), the former most senior civil servant in the Foreign Office 'eviscerated' the Prime Minister on Tuesday
Indeed, it is even said that Trump's presidential transition team, appalled at the prospect of Mandelson getting the job, repeatedly tried to intervene on Pierce's behalf.
But by the beginning of November 2024, the pressure was beginning to tell on Barton, who did not even have the support of David Lammy, another of Mandelson's many backers.
Barton, 62, had been planning to retire in 2026 after 40 years of service. However, he agreed to go some months early – in a decision that entitled him to a 'voluntary exit compensation payment' of £262,185 under the terms of the Civil Service Compensation Scheme. The money was in addition to his 2024–25 salary of £240,000 including bonuses.
As a result, the appointment of Mandelson was signed off (weeks after the King had been informed) on the last day of the Biden administration – not by Barton, but by his successor and friend Sir Olly Robbins.
The entire controversy will be picked over by the Foreign Affairs Select Committee next week when it will interview Morgan McSweeney and Barton on successive days.
'When Philip left, he feared the Mandelson thing would blow up,' says my Whitehall source. 'How right he was.
'Olly Robbins took over, knowing it was job done about Mandelson, and the vetting process seemed to be all about how quickly could they get it done.'
Barton's supporters say that he was forced out because he was standing in the way of the Mandelson appointment and the Foreign Affairs Select Committee is certain to ask him if that is true.
If he confirms that he jumped before he was pushed, the news that not one but two permanent secretaries were thrown under the bus to ensure Sir Keir Starmer's appointment of Mandelson went ahead will only ramp up the pressure on our already beleaguered Prime Minister.