More than 200 files on overseas trips made by the former Prince of Wales stretching back to the 1970s have been censored from public view.
Written by British diplomats stationed around the world, they are believed to contain extensive commentary on the new monarch’s past life as heir to the throne.
Over half of the records contain references to his first wife, the late Princess Diana.
The papers cover visits by the couple to almost every continent, including their honeymoon cruise around the Mediterranean on the royal yacht.
One file about their 1983 visit to Australia, which Netflix featured in The Crown, will remain classified until 2070. By then Charles would be 121 years old.
Under UK transparency laws, government departments must release their records to the public between 20 and 30 years after they were written.
Civil servants can keep files closed for longer only with the approval of the Lord Chancellor, the government’s justice secretary Brandon Lewis.
Some censorship decisions are scrutinised by an advisory council, whose members have at times included an aide to King Charles.
Only the titles of the 212 sealed files are shown on a database hosted by the National Archives, which was analysed by Declassified.
View the full list of secret Foreign Office files on King Charles III
Historians say the trove could shed light on the role Charles played in British foreign policy during the end of Empire and the Cold War.
One of the oldest documents contains a Foreign Office briefing for Charles’ visit to the Bahamas during their independence ceremony from Britain in 1973.
Although his mother remained head of state, the Caribbean nation now plans to hold a referendum on becoming a republic.
‘No reason to be kept secret’
Norman Baker, a former government minister, told Declassified: “It is outrageous that so many files from so long ago regarding Charles have been kept from public view. It is contrary to the spirit of the Freedom of Information Act and the Public Records Act.”
Baker, who wrote the book And What Do You Do?: What The Royal Family Don’t Want You To Know, added: “I am calling for the National Archives to conduct an investigation of this matter with a view to releasing some or nearly all of these documents.
“There’s no reason for these to be kept secret. The normal excuse given is that it’s to uphold the dignity of the crown. But the dignity of the crown is upheld by them not behaving in an undignified manner.”
Many of the secret papers relate to trips by Charles to repressive Arab monarchies in the Middle East. Diplomats appeared to notice Charles had an interest in Islamic culture and considered how to cultivate his curiosity.
A file from 1989 is titled: “Interest in the Gulf by the Prince of Wales: potential benefits for relations between the Gulf and the UK.” Charles is known to have visited Gulf regimes like Saudi Arabia at crucial moments in negotiations on arms deals worth billions of pounds to British business.
In preparation for a visit to the Gulf in 1986, diplomats compiled “briefs on Defence Sales and Conversational topics” to guide Charles on discussing arms deals. The paper is sealed until 2049.
Diplomats even drew up a document titled “Visit by the Prince of Wales to Israel” in 1986. Its existence is curious because Charles has never travelled to the country.
He privately expressed criticism of Israel and the “Jewish lobby” in a letter sent to a friend that year while journeying through other parts of the Middle East.
Another Foreign Office file concerns the King’s “interest in Romania” after the fall of its Communist dictatorship in 1989.
Charles, who is distantly related to Romanian royalty, has purchased property and swathes of land in Transylvania – home of the mythical vampire Dracula.
Professor Philip Murphy, an historian and author of Monarchy and the End of Empire, commented: “The palace is terribly risk averse. They don’t really understand a lot of this is already in the public domain. All we’re really adding is thick archival description.
“That’s why there’s sort of a duty to push back against them, because I don’t think officials are prepared to do that and politicians aren’t – they’re worried about getting honours. So historians and journalists have really got to do that.”
Murphy believes it was easier in the 1990s for him to access records on the royal family, as a Conservative minister William Waldegrove introduced a “gentleman’s agreement” to relax the classification of historical files.
Tony Blair’s Labour government then passed the Freedom of Information Act, which codified a right of access to certain records. Ironically, Murphy said: “The shutters came down again because the palace panicked.”
Historians face a further struggle to access the royal family’s own records, which are kept in a tower at Windsor Castle where researchers should be allowed access to papers from previous reigns.
Baker commented: “It is very difficult to get in, even for me as a Privy Counsellor. There’s only a small room, which holds about four researchers at any one time, which is absurd given the size of Windsor Castle.
“There’s no index to what is available. You have to ask for things and hope that by throwing a dart at a dart board with a blindfold on, you hit something.”
Murphy agreed, saying: “They make it quite tricky for historians to work with them.”
In theory, letters from Queen Elizabeth II’s own collection are now eligible to be transferred to the Royal Archives following her death last month aged 96.
Declassified asked to view letters between the late Queen and Sultan Qaboos of Oman, an Arab autocrat backed by Whitehall.
However, Royal Archives manager Bill Stockting told Declassified they have “not yet accessioned the records of Her late Majesty’s reign” and could not say when they would become available.
A Foreign Office spokesman said: “Under the terms of the Public Records Act 1958, historical records can be legitimately withheld.”
The National Archives declined to comment.