Buckingham Palace allegedly received emails as far back as 2020 which showed Andrew Mountbatten-Windsor was sharing classified information with business contacts while serving as a UK trade envoy.
The revelation comes from court documents which describe a stack of 30,000 emails, including some which were personally sent by the former Duke of York.
The collection of emails was handed over six years ago to the Lord Chamberlain, the royal household's most senior officer, from a third party, the BBC reports.
But the palace states it is "not possible" to provide any comment on what happened to the archive, as a police investigation into Mr Mountbatten-Windsor's activities continues.
Thames Valley Police reissued its appeal for potential witnesses to come forward with information last week, after Mr Mountbatten-Windsor was arrested in February on suspicion of misconduct in public office.
A legal dispute between two businessmen appears to confirm the email archive’s existence.
High Court papers describe a row involving Jonathan Rowland, former chief executive of Banque Havilland, and his father David, with retail tycoon Kevin Stanford - who gained possession of the emails from Mr Rowland’s account.
In the legal dispute, it was claimed the emails were illegally obtained from one of Mr Rowland's business associates in 2013 and in more than one message, Mr Stanford said he had handed over a copy of the archive to Buckingham Palace.
A High Court ruling from June 13, 2022, states: “In an email... dated 10 July 2020, Mr Stanford referred to having delivered to Buckingham Palace ‘material (which includes the archive)’ …”.
A separate judgment in April 2021, states: "Mr Stanford’s emails also make it clear that he provided a copy of the archive to the Lord Chamberlain in May 2020."
The cache of emails spans a time period up to June 2013 and although their contents is not known, one incident involving Mr Rowland may be linked to them.
In February, The Telegraph published a story that Andrew Mountbatten-Windsor had passed on a confidential Treasury briefing to Jonathan Rowland in 2010, while he was serving as trade envoy.
At the time, Britain and Iceland were mired in a row over British deposits lost during the 2008 banking crisis. Andrew's deputy private secretary Amanda Thirsk received a briefing note from a civil servant at the Treasury a week after his official visit to Iceland, and passed it to him.
Email correspondence shows Andrew then passed the confidential message over to his close friend, Mr Rowland, whose Banque Havilland enterprise had purchased assets from a failed Icelandic bank Kaupthing around 12 months earlier.
Andrew's message to Mr Rowland stated he thought the information might be useful "before you make your move".
Mr Rowland later confirmed confirmed to the BBC that the emails concerning Icelandic banks originated from his account and were a part of legal proceedings.
The Rowland family and Banque Havilland were later hit with sanctions from UK and EU regulators, and Mountbatten-Windsor's emails concerning the group's activities have attracted significant attention.
When asked if it had access to the 30,000 emails referred to by the High Court in 2021, Thames Valley Police said it could not comment on specific information.
The force stated: "We are aware of the allegations circulating in the public domain and encourage anyone with relevant information to get in touch."
A government spokesperson added: "We are fully cooperating with Thames Valley Police, and last week we published documents about the creation of the role and Andrew Mountbatten-Windsor's appointment in 2001."
Buckingham Palace, Kevin Stanford and Jonathan Rowland have been approached for comment.
