The Duke of Edinburgh was among the figures referenced in a fresh tranche of documents released as part of the so-called "Mandelson files".
These were published by the Government on June 1 following a parliamentary demand for greater transparency surrounding Lord Mandelson's appointment as Britain's ambassador to Washington.
The latest release, comprising more than 1,500 pages of emails, messages and official documents, offers an unprecedented glimpse into discussions within Government and the Foreign, Commonwealth and Development Office (FCDO) during Lord Mandelson's controversial appointment process.
The publication follows months of political pressure and scrutiny surrounding his vetting, business connections and links to the late financier Jeffrey Epstein.
Among the documents is an email exchange from December 31, 2024 discussing Britain's representation at the state funeral of former US President Jimmy Carter.
At 4.47pm, Robert Tinline, the FCDO's Director for the Americas, wrote to Lord Mandelson: "Assume you're aware it's looking like Duke of Edinburgh and Gordon Brown going to the Carter funeral for the UK."
Just 16 minutes later, Lord Mandelson replied: "Edward and Gordon will get along famously."
The exchange appears to have been a light-hearted reference to the UK's delegation for the funeral, which took place in Washington, DC on January 9, 2025.
The publication follows months of political pressure and scrutiny surrounding Peter Mandelson's vetting, business connections and links to the late financier Jeffrey Epstein
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The Duke of Edinburgh, representing King Charles III, attended the funeral service at Washington National Cathedral alongside former Prime Minister Gordon Brown.
Prince Edward was the most senior British royal present and paid tribute on behalf of the King.
Mr Carter, the 39th President of the United States and a Nobel Peace Prize laureate, died on December 29, 2024 at the age of 100.
His state funeral drew an extraordinary gathering of political leaders and dignitaries from around the world.
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Prince Edward sat in front of Gordon Brown at Jimmy Carter's funeral
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All five living US presidents, Joe Biden, Donald Trump, Barack Obama, George W. Bush and Bill Clinton, attended the ceremony.
International guests included Canadian Prime Minister Justin Trudeau, UN Secretary-General António Guterres and representatives of numerous foreign governments and royal houses.
The publication of the Mandelson files has reignited debate over the circumstances of Lord Mandelson's diplomatic appointment.
Documents released earlier this year revealed concerns raised during the security-vetting process regarding some of his international contacts and business interests.
Prince Edward attended the funeral as a representative of King Charles III
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Other files disclosed internal discussions about his appointment and communications between senior Labour figures.
Lord Mandelson has repeatedly denied wrongdoing.
He has rejected allegations that he acted improperly in relation to Jeffrey Epstein and has denied any misconduct connected to his appointment process.