Estranged father Thomas Markle could be called on to testify at the high court against his daughter Meghan as part of her ongoing legal action against the Mail on Sunday.

Reports have emerged suggesting that the paper’s defence is reliant on Thomas’ account.

The Duchess of Sussex is currently suing the newspaper for breach of copyright, invasion of privacy and misuse of personal data after it published excerpts from a letter that she sent to Thomas about how he was treating her.

Mail on Sunday filed its 44-page long defence at the high court on Tuesday arguing that the Duchess and other royals rely on publicity about themselves and their lives to maintain the privileged positions they hold”. It also argues that Meghan could not “have a reasonable expectation of privacy that the contents of the letter were private and would remain so”.

“There is a huge and legitimate public interest in the royal family and the activities, conduct and standards of behavior of its members,” the filing said. “This extends not merely to their public conduct, but to their personal and family relationships because those are integral to the proper functioning of the monarchy.”

The paper is targeting all of Meghan’s claims, including where she says that she had not courted publicity for her relationship with her father. The paper has countered this saying that Meghan hasn’t denied authorising her friends to speak about the relationship for an article with US magazine People.

Mail on Sunday has dismissed the copyright claim by arguing that the letter was not an original literary work while also saying that Meghan and her team “briefed” the media against Thomas to obtain favourable coverage.

"The privacy allegation, which is being made by Meghan in this particular case, is that there was also an invasion of her privacy by the revelation of this letter and the information about their personal relationship," media lawyer Mark Stephens told the BBC.

"Effectively, Thomas Markle and therefore the Mail on Sunday's defence is that because it is alleged Meghan and her team were out briefing against Thomas Markle — he had a right of reply, he had a right to defend himself, he had a right to put the record straight and it's much less clear that she wins that cleanly or at all".

The legal proceedings are being funded by the Duke and Duchess of Sussex privately and any proceeds will be donated to an anti-bullying charity.