Meghan Markle's estranged father Thomas Markle is willing to testify against his daughter in a court case the Duchess of Sussex has brought against a British tabloid newspaper.

Key points: Lawyers for the Duchess filed the court claim alleging misuse of private information

Lawyers for the Duchess filed the court claim alleging misuse of private information The newspaper's defence argues that Thomas Markle released the letter to the press

The newspaper's defence argues that Thomas Markle released the letter to the press The Duchess of Sussex made her first public appearance at a women's centre in Vancouver

Defence documents filed in the High Court in London by the Mail on Sunday indicate that evidence from Thomas Markle will be used if the matter reaches trial.

The suit relates to a private letter written by the Duchess of Sussex to her father following her marriage to Prince Harry in 2018 and published by the Mail on Sunday.

Lawyers for the Duchess filed the court claim in October 2019, alleging misuse of private information, infringement of copyright and breach of the Data Protection Act 2018.

Meghan's father became the focus of intense media scrutiny in the lead-up to the 2018 wedding at Windsor Castle after it emerged that he had staged photos with a paparazzi photographer.

He missed his daughter's wedding after undergoing heart surgery.

In the 44-page document lodged in the High Court in London outlining the newspaper's defence, the paper argues that Thomas Markle released the letter to the press to expose false claims against him.

In addition, the documents allege that the Duchess of Sussex and her team had "briefed" the media against Thomas Markle to obtain favourable coverage and that there was a "huge and legitimate" public interest in publishing extracts of the letter.

"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".

Meghan's first appearance in Canada

The Duchess of Sussex did not attend the family summit as she was in Vancouver with son Archie. ( Facebook: Downtown Eastside Women's Centre )

The lawsuit follows months of acrimony between the British press and the Royal couple, who have expressed unhappiness with the press coverage they have received.

At the time, Prince Harry said the publication of the private letter by the Mail of Sunday was released in "an intentionally destructive manner".

"It will end up surely either with a mucky compromise in which they do not win or they go to court and it will be an extraordinary event … with Meghan Markle's father giving evidence against her, possibly Prince Harry being dragged into court as well," Roy Greenslade, Former Editor of the Daily Mirror, told the BBC.

"I would've thought that the Royal Family as a whole will suffer if this becomes a public event, which is what it would be in a court action."

Other commentators thought it would not come to that.

"The Palace really cannot run the risk of putting her [Meghan Markle] into the witness box because what if she were not believed or in some way found to be not a witness of truth? It just beggars belief," Mr Stephens said

"And of course the editor of the Mail on Sunday has absolutely no incentive to settle this. As far as he's concerned, he's died and gone to heaven because every time this case comes up, he gets another story.

"Every single element of this is another story and more publicity for the Mail on Sunday. So it doesn't matter to him what this costs at all."

The Queen agreed to a transition period for the couple as they step back as senior royals. ( AP: John Stillwell/Pool Photo, file )

It follows days of intense scrutiny around the Royal Family after Prince Harry and Meghan announced they planned to step away from their roles as senior royals.

The revelation surprised even those inside Buckingham Palace, but following a summit at the Queen's estate in Sandringham earlier the week, the Queen agreed to a transition period for the pair.

"Although we would have preferred them to remain full-time working Members of the Royal Family, we respect and understand their wish to live a more independent life as a family while remaining a valued part of my family," the Queen said in a statement.

Meghan was not present at the crisis talks, but instead earlier returned to Canada, where the couple intend to split their time.

The Duchess of Sussex made her first public appearance at a women's centre on Tuesday (local time) in Vancouver.

"Look who we had tea with today!" the centre posted alongside a photo on Facebook.

Questions remain over how the new Royal arrangement will be financed, who will foot the bill over the pair's security detail and whether the pair will keep their royal titles.

Prince Harry will make his first public appearance since the furore at Buckingham Palace on Thursday (local time).