The handwritten letter to David Lammy, then Britain’s foreign secretary, conceded that the country’s ambassador to the United States during the second Trump administration would require “superhuman skills” as well as luck.

But Peter Mandelson thought he knew just the right candidate.

“If you were minded to appoint me I would make sure you never regret it,” he pledged, in the November 2024 letter.

It was a promise he could not keep.

The decision to appoint Mr. Mandelson as Britain’s top envoy to the United States would become one of the biggest regrets of Britain’s beleaguered prime minister, Keir Starmer. He fired the ambassador in September 2025 over his links to Jeffrey Epstein and apologized to Parliament in April, acknowledging, “I should not have appointed Peter Mandelson.”

On Monday more than 1,500 pages of previously unpublished documents relating to the appointment were released, including dozens of pages of private text messages between the then-ambassador and senior Labour government officials. They lift the lid on the thoughts of Mr. Mandelson, one of the most mercurial, effective and longstanding operators in British politics, as well as revealing the doubts of some senior Labour figures about the direction of their own government.

In emails and text messages the former ambassador, who took up his diplomatic post in February 2025, shared his insights into President Trump’s administration, while commenting on the failings of Mr. Starmer’s government back home.

There were also glimpses into the demands of diplomacy in Washington. A series of emails between Foreign Office and embassy officials discussed a gift for Mr. Trump that the administration had apparently asked Britain to procure.

“Washington, as I’m sure you know, is clear that one of the gifts that would mean the most to the President would be a red dispatch box with the gold crest and lettering mimicking a U.K. Government Ministerial box but with ‘President of the United States’ inscribed upon it,” the email read.

Dispatch boxes are red leather, lead-lined cases that hold official British government papers, with origins in the Tudor era. These days, they are found in the hands of the prime minister, other government ministers and the British monarch, who receives a daily red box containing state documents that require royal review. Mr. Starmer did eventually present a red box to Mr. Trump during the president’s state visit to Britain in September 2025.

The files, the release of which was forced by opposition politicians, also featured several embarrassing exchanges for Mr. Starmer, including:

• A WhatsApp exchange with a senior cabinet minister, Pat McFadden, in which Mr. Mandelson wrote that the difficulties of Mr. Starmer’s government in setting a direction stem “from the top and Keir lacks verve as does the Cabinet as a whole. People’s heads are broadly in the right place but you need more people who can execute.”

• In another WhatsApp message, Mr. Mandelson described going into Downing Street and finding it “beleaguered and bereft,” adding: “It requires complete revamp and infusion of purpose and confidence to get anywhere.” Key staff there do not “really know what Keir thinks or wants,” Mr. Mandelson wrote. “In fact most of them don’t think Keir knows what he wants.”

• In a reply to one of Mr. Mandelson’s messages, Mr. McFadden referred to the mood among Labour lawmakers who were appealing for more welfare spending, saying, “Every meeting I have is ‘Who can we tax in order to pay benefits to others,” adding: “They’re asking the wrong questions.”

Mr. Mandelson lost his job last year after leaked emails showed the depth and extent of his friendship with Mr. Epstein, the convicted sex offender, kindling a political crisis in Britain.

In March, a preliminary release of documents showed that British vetting officials had recommended against granting top-level security clearances to Mr. Mandelson before he was made Britain’s top envoy to the United States. But the recommendation was overruled by Olly Robbins, the top official at the Foreign Office.

Mr. Starmer has said that he was lied to by Mr. Mandelson over the extent of his ties to Mr. Epstein. Mr. Starmer has also said that he was not told about the security clearance recommendation.

The furor over Mr. Mandelson’s appointment prompted the firing of Mr. Robbins in April and helped to destabilize Mr. Starmer’s position as prime minister and as leader of the governing Labour Party.

Mr. Mandelson is under criminal investigation on allegations of misconduct in public office after allegations that he passed confidential government information to Mr. Epstein while serving in a previous Labour government, in 2009 and 2010. Mr. Mandelson has denied criminal wrongdoing and he is cooperating with the police. He was arrested and released on bail in February and has not been charged.

The government promised that all relevant documents would be published, apart from those that the police had asked to be held back while they conduct their investigation.

However, some documents were heavily redacted on national security or international relations grounds, or to remove the names of junior officials. That provoked complaints from some opposition politicians.

Mr. Starmer’s political fortunes worsened last month after poor local election results and the resignation of a senior cabinet minister, Wes Streeting, prompting speculation that the prime minister could face a leadership challenge within months. One of the main contenders, Andy Burnham, currently the mayor of Greater Manchester, in northwestern England, is running in a special election to return to Parliament.

The documents released on Monday reveal that doubts about Mr. Starmer’s ability to survive in Downing Street were being expressed almost a year ago when the prime minister was forced into a retreat over welfare policy.

In a WhatsApp exchange with Mr. McFadden, Mr. Mandelson said that if a vote in Parliament was lost, “I am not sure that Keir survives that.” Mr. McFadden agreed that if Mr. Starmer suffered a parliamentary defeat, axed the welfare bill or decided to “gut” it, that would “destroy his authority,” adding that he meant: “Keir’s authority.”

Mr. Mandelson responded: “Yes I am afraid so.”

Mr. Starmer ultimately survived his retreat over the welfare bill but now faces greater political peril than ever, in part because of his fateful decision to give Mr. Mandelson the job he craved in Washington.