The Government has already spent an estimated £300,000 of public money on the case, which is being fought by Andrew Lownie, the author and historian.

For the past four years, Mr Lownie has been battling to gain unfettered access to the Mountbatten Archive, which was bought by the University of Southampton in 2010 using £2.8 million of public money.

In 2019, the Information Commissioner found in his favour and ordered the release of the entire archive. However, the Cabinet Office appealed and has continued to block access to certain passages in the diaries, some of which date to the Thirties. That appeal hearing is scheduled to start next week.

Part of the evidence before the hearing will concern the Royal household’s role in vetting the material.

In 2011, Prof Chris Woolgar, the chief archivist of the Broadlands Archive (named after Mountbattens’ country home) at the time, wrote to the Cabinet Office proposing to censor certain parts of the diaries that referred to the Royal family and the partition of India, which happened while Lord Mountbatten was the country’s viceroy.

He said in an email to the Cabinet Office: “I don’t believe these should be available to researchers, possibly from as far back as the mid-1930s, given their many references to the Royal family (which I can spot).”