Thomas Markle could be called to give evidence against his daughter Meghan, the Duchess of Sussex, in the UK's high court, the Daily Mail reported on Wednesday.
The Daily Mail is the sister newspaper to the Mail on Sunday, which is being sued by the Duchess over its publication of a private letter she wrote to her estranged father criticising his treatment of her.
Documents filed to the high court by the Mail on Sunday on Tuesday, which have been seen by The National, argue that there is a "huge and legitimate" public interest in publishing the letter.
Meghan's half-sister Samantha Markle told the BBC that her father would tesitfy in the case against her "if he is called".
The development in the legal case comes a week after Meghan and her husband Prince Harry announced they would be stepping back as senior members of the British royal family.
Law firm Schillings, acting on behalf of Meghan, filed a High Court claim in October alleging misuse of private information, copyright infringement and breach of personal data after the newspaper published extracts from a letter she sent to her father in August 2018.
In their 44-page defence filing to the court, the Mail on Sunday claims that Meghan and other members of the royal family "rely on publicity about themselves and their lives in order to maintain the privileged positions they hold and promote themselves". It adds that the Duchess could not "have a reasonable expectation of privacy that the contents of the letter were private and would remain so".
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Britain's Queen Elizabeth II, Meghan the Duchess of Sussex and Prince Harry watch a flypast of Royal Air Force aircraft pass over Buckingham Palace in London. AP Photo -
Britain's Prince Harry and Meghan, Duchess of Sussex leave after visiting Canada House in London after their recent stay in Canada. AP Photo -
Britain's Prince Harry and Meghan, Duchess of Sussex, arrive to greet members of the public in Kingfisher Bay on Fraser Island in Queensland, Australia October 22, 2018. Reuters -
Britain's Prince Harry and Meghan, Duchess of Sussex meet with a local surfing community group, known as OneWave, raising awareness for mental health and wellbeing in a fun and engaging way at Bondi Beach in Sydney, Australia. AP Photo -
Britain's Prince Harry and Meghan, Duchess of Sussex hold their baby son Archie during a photocall in St George's Hall at Windsor Castle, in Berkshire, Britain May 8, 2019. Reuters -
Meghan, Duchess of Sussex, visits Auwal Mosque on Heritage Day with Prince Harry, Duke of Sussex, during their royal tour of South Africa on September 24, 2019 in Cape Town, South Africa. Getty Images -
Prince Harry, Duke of Sussex and Meghan, Duchess of Sussex, pose for a photo with the New York Yankees before their game against the Boston Red Sox at London Stadium on June 29, 2019 in London, England. The game was in support of the Invictus Games Foundation. Getty Images -
Prince Harry, Duke of Sussex and Meghan, Duchess of Sussex, dance as they visit a Justice Desk initiative in Nyanga township, during their royal tour of South Africa on September 23, 2019 in Cape Town, South Africa. Getty Images -
Prince William, Duke of Cambridge and Catherine, Duchess of Cambridge, Prince Harry, Duke of Sussex and Meghan, Duchess of Sussex attend a service marking the centenary of WW1 armistice at Westminster Abbey on November 11, 2018 in London, England. Getty Images -
Prince Harry, Duke of Sussex and Meghan, Duchess of Sussex, visit an exhibition of Tongan handicrafts, mats and tapa cloths at the Fa'onelua Convention Centre on October 26, 2018 in Nuku'alofa, Tonga. Getty Images -
Prince Harry, Duke of Sussex and Meghan, Duchess of Sussex, meet with local children as they arrive at Dubbo Airport on October 17, 2018 in Dubbo, Australia. Getty Images -
Britain's Prince Harry, Duke of Sussex, and his new wife Meghan, Duchess of Sussex, leave from the West Door of St George's Chapel, Windsor Castle, in Windsor on May 19, 2018 in Windsor, England. Getty Images -
Prince Harry, Duke of Sussex, and Meghan, Duchess of Susse, arrive for a visit to "Justice desk", an NGO in the township of Nyanga in Cape Town, as they begin their tour of the region on September 23, 2019. AFP -
Britain's Prince Harry, Duke of Sussex, chats with Disney CEO Robert Iger as Britain's Meghan, Duchess of Sussex, embraces US singer-songwriter Beyoncé as they attend the European premiere of the film The Lion King in London on July 14, 2019. AFP -
Britain's Prince Harry and Meghan, Duchess of Sussex, joined by her mother, Doria Ragland, show their new son to Queen Elizabeth II and Prince Philip at Windsor Castle, Windsor, England. AP Photo
Dismissing the copyright infringement claim, the newspaper says in its filing that Mr Markle gave them the letter after an article defending the Duchess appeared in US magazine People which made reference to the letter.
The article quoted five unnamed friends of the Duchess, which the Mail on Sunday claims Meghan herself must have "authorised or acquiesced to the disclosure of the fact and nature of the Letter to People magazine".
The Mail on Sunday said that Mr Markle "had a weighty right to tell his version of what had happened between himself and his daughter including the contents of the letter".
The documents also include the content of text messages from Mr Markle to Meghan sent in the days before her wedding to Harry in May 2018. The publication of the highly personal messages has led commentators to suggest that the newspaper’s court filing was made with cooperation from Mr Markle.
The Daily Mail said Mr Markle, a 75-year-old former Hollywood lighting director, "expected to be called as a key witness" should the case be brought to court.
Meanwhile, Meghan was seen publicly for the first time since returning to Canada last week at a women's centre in Vancouver.
The Downtown Eastside Women's Centre posted a photograph to Facebook of the Duchess with staff on Tuesday, adding that she had visited "to discuss issues affecting women in the community".
Her husband Harry is still in the UK, where he is expected to attend an official event at Buckingham Palace on Thursday.
Harry had talks with his grandmother Queen Elizabeth on Monday after which the British monarch released a statement saying she accepted the Sussexes’ decision to step back as senior royals.
