Court documents show Buckingham Palace received an archive containing 30,000 emails six years ago, revealing Andrew Mountbatten-Windsor was passing confidential Government information during his tenure as Britain's trade envoy.
The substantial email collection, detailing the former prince's contentious financial activities, was delivered to the Lord Chamberlain, the Royal Household's most senior official, in 2020.
These communications had been extracted from the account of one of Mr Mountbatten-Windsor's private business associates.
When approached, the Palace said: "Since there is an ongoing police enquiry concerning Mr Mountbatten-Windsor, it is not possible to provide any comment on these matters."
Thames Valley Police last week renewed their appeal for witnesses following Mr Mountbatten-Windsor's arrest on suspicion of misconduct in public office.
High Court records from April 2021 confirm a copy of the archive was provided to the Lord Chamberlain, which was held by Lord Peel, in May 2020, with a subsequent ruling from June 2022 referencing correspondence confirming delivery to the Palace.
Earlier this year, the Telegraph revealed emails detailing that Mr Mountbatten-Windsor had obtained a confidential Treasury briefing in 2010 concerning difficulties within Iceland's banking sector.
Allegedly, he forwarded this sensitive information to Jonathan Rowland, advising him to review it "before you make your move".
Court documents have disclosed that Buckingham Palace received an archive containing 30,000 emails six years ago, which revealed Andrew Mountbatten-Windsor was passing confidential government information to during his tenure as Britain's trade envoy
| GETTYMr Rowland's father, David Rowland, had recently acquired the Luxembourg division of the collapsed Icelandic institution Kaupthing, which was subsequently renamed Banque Havilland.
This bank would later face sanctions from regulatory authorities in both Britain and the European Union.
Jonathan Rowland has previously confirmed to the BBC that these published communications originated from his account and formed part of legal proceedings. It is not known what happened to the emails shared with the Palace.
York Central MP Rachael Maskell has demanded a public inquiry into the affair, declaring on the BBC that "the system built around the Royal Household has to be reviewed".
LATEST ROYAL NEWS:
York Central MP Rachael Maskell has demanded a public inquiry into the affair, declaring on the BBC that 'the system built around the Royal Household has to be reviewed'
| PAThe Labour parliamentarian warned: "The web grows ever darker and that is why we have got to address the issue of unaccountable power and also the abuse of power in high office."
Mrs Maskell advocated for establishing a joint committee comprising members from both the House of Lords and House of Commons to scrutinise the Royal Household's operations.
Ailsa Anderson, who served as press secretary to the late Queen Elizabeth II, described the revelations as "absolutely horrendous" and "another nail in the coffin".
Mrs Anderson acknowledged the police inquiry must proceed unimpeded whilst noting the Royal Household had responded swiftly.
Thames Valley Police continue their investigation following Mr Mountbatten-Windsor's arrest in February, though a spokesman declined to confirm whether officers have obtained access to the 30,000 emails referenced in High Court proceedings five years ago.
He has always denied any wrongdoing or any personal gain from his role as trade envoy.
The force stated: "We are aware of the allegations circulating in the public domain and encourage anyone with relevant information to get in touch."
King Charles has maintained a firm stance since his brother's arrest, declaring: "The law must take its course."
A Government spokesperson confirmed full cooperation with Thames Valley Police.