Earlier hearings at the High Court made reference to the so-called Palace Four: the Duchess’s former private secretary Samantha Cohen, and former press secretaries Jason Knauf, Christian Jones and Sara Latham.

Documents released in the Court of Appeal this week also mention Simon Case, the Duke of Cambridge’s former private secretary who is now Cabinet Secretary and head of the civil service.

The obligation to provide information to the court or later appear as witnesses would override the customary discretion of members of the Royal household. They are thought to have signed non-disclosure agreements about their work at the palace, which could now be rendered useless.

Keith Mathieson, ANL’s solicitor, told the court he had contacted the solicitor representing the “Palace Four” on numerous occasions, eventually being told in December 2020 that they “had relevant evidence” about the letter and “whether or not the claimant [Meghan] anticipated that the letter might come into the public domain”.

“However, the letter did not elaborate any further as to what that evidence might be,” he added.

Mr Knauf, who went on to take legal advice from a different firm, has now submitted written evidence including emails and text messages to and from the Duchess of Sussex.

They show her telling him she had “obviously” written the letter to Mr Markle “with the understanding that it could be leaked”, and included emails with both the Duke and Duchess of Sussex in which they authorised him to brief the authors of Finding Freedom.

The messages cover just a few key moments in the Sussexes’ time at Kensington Palace and later Buckingham Palace.

Mr Knauf left his role in March 2019, moving to be chief executive of the Cambridge’s Royal Foundation, and he has said he cannot shed light on events relating to the Sussexes after his departure.

Finding Freedom was published in August 2020, after the Duke and Duchess moved through two more press secretaries and left for a new life in California.

‘Masses of material is going to be searched’

Mark Stephens, a media law expert and partner at Howard Kennedy, said it was likely that internal emails and text messages between palace staff would be produced in court if the case moved to a full trial.

“It’s almost certain that masses of that material is going to have to be searched to look for relevant material,” he said.

But he added: “I think it’s almost a racing certainty that whilst it will be handed over the Mail on Sunday’s lawyers, the judge might put in place reporting restrictions to prevent it being used in news reporting.”

Mr Stephens said that because the Duke and Duchess of Sussex were essentially estranged from the Royal family, they may well no longer have access to royal systems including emails, meaning a judge would have to make a third party disclosure order directly against the palace.

The Duchess’s own witness statement claimed she was bracing to search her own email and text archives for 177 terms at the insistence of ANL, although conceding that these were later narrowed down.

Paperwork dating from September 2020 from the Duchess’s team said ANL was seeking “all disclosures and communications about her relationship with her father over a period of 18 months” with 40 search terms.

In the event, the searches were not provided to the court after the Duchess won her case – of breach of privacy, copyright and data protection – in a High Court summary judgment which found the publication of extracts of her letter to Mr Markle to be unlawful.

The Mail on Sunday is seeking to overturn that summary decision, arguing the matter should go to trial.

After a three-day hearing this week, the Court of Appeal judges will return their findings within the next few weeks.

Given the involvement of a member of the Royal family, the Duchess of Sussex’s case is unprecedented in the modern British courts.

In its closest equivalent, in 2006, the Mail on Sunday lost an appeal over the publication of extracts from the Prince of Wales’s private diaries describing the handover of Hong Kong in 1997, with a summary judgment ruling the newspaper had breached his copyright and privacy.

Mr Stephens said the current case was the first major legal dispute involving the Royal family in which electronic communications could play a vital role.

“The Charles case predates Whatsapp, Messenger and that kind of thing, so it isn’t an area that has been gone into before,” he said.

Buckingham Palace did not comment.