The potential perils for Meghan, Duchess of Sussex in her high-stakes legal battle against the Mail on Sunday have become clear after the newspaper filed a detailed legal defence hinting at the future release of royal text messages.

The case revolves around allegations of breach of copyright, invasion of privacy and misuse of personal data after the newspaper published excerpts from a letter she sent to her father complaining about his treatment of her.

But documents filed at the high court revealed the paper will argue that Meghan is a “major public figure, whose fitness to perform royal duties on behalf of the Crown and to be the recipient of public money is a proper matter for public scrutiny”.

In the defence, lawyers for Associated Newspapers state she “did not have a reasonable expectation of privacy” for the letter’s contents because there was “no mutual understanding” between the duchess and her father that the correspondence should remain secret.

The defence states that the newspaper will seek disclosure from Meghan of her exchanges with her friend Jessica Mulroney about allegedly providing information to the media to influence what was published.

The newspaper’s submission also refers to a text message sent to Thomas Markle signed, ‘Love M and H’, which “read as if it was from Prince Harry … admonishing Mr Markle for talking to the press and telling him to stop and accusing Mr Markle of causing hurt to his daughter”.

The fact that the letter was handwritten in such a clear manner is further evidence that it was sent by Meghan “with a view to it being read by third parties and/or disclosed to the public”, the Mail on Sunday’s lawyers maintain.

“The letter appears to have been being immaculately copied out by [Meghan] in her own elaborate handwriting from a previous draft. There are no crossings-out or amendments as there usually are with a spontaneous draft. It is to be inferred also from the care [she] took over the presentation of the letter that she anticipated it being disclosed to and read by third parties.”

On Wednesday, Thomas Markle’s daughter from his first marriage, Samantha, confirmed that he would testify in the court case. “If he is called, he will come,” the duchess’s half-sister told the BBC.

Schillings, a law firm specialising in libel and “reputation management”, launched the legal action for Meghan against the Mail on Sunday in September. The action is due to be heard in the chancery division of the high court. No hearing date has yet been set. Mr Justice Warby is expected to be in charge of the case.

Legal experts questioned the strength of the newspaper’s defence, but also whether Meghan and Prince Harry would want to put themselves through such a painful courtroom ordeal of cross-examination by a top-flight QC and demands for disclosure of personal documents.

Hugh Tomlinson QC, chair of the privacy lobby group Hacked Off, who acted for Prince Charles in 2006 against the Mail on Sunday to halt the publication of his diaries, told the Guardian that the law accepts that even public figures are entitled to private lives. “If a newspaper is going to rely on [a] public interest [defence], then it must demonstrate that there’s something serious or substantial that needs to be exposed to the public,” he said.

“Private lives need to be kept private and you have to have a very strong reason for [revealing details] which goes beyond the fact that they are quite well known figures.”

Mark Stephens, a libel and privacy expert at the law firm Howard Kennedy, said Meghan would “undoubtedly win the case”, but wondered whether it would ultimately be worth the “enormous price”. While she would win what he described as a “tiny legal battle”, he warned that she and her husband would comprehensively lose the “much broader war”.

He added: “The point about copyright is that it gives control to the person who has the copyright – in this case Meghan – and in those circumstances she can allow people to use the copyright or not use the copyright.

“What can’t happen is that Thomas Markle cannot unilaterally decide, nor indeed can the Mail on Sunday unilaterally decide, that they want to publish this material,” he told the BBC’s Today programme.

The detailed Mail on Sunday response has been drafted by Anthony White QC.

As well as Schillings, Meghan is being represented by a senior barrister at 5RB chambers who has previously acted for Prince Harry’s mother, Princess Diana, as well as Tony Blair and Donald Trump.

While the legal battle raged on in the UK, Meghan was in Canada with the couple’s baby son, Archie, and their dogs. She visited the Downtown Eastside Women’s Centre in Vancouver, attending a meeting on Tuesday to discuss issues affecting women in the community, of which the centre posted a photograph on its Facebook page.

Buckingham Palace is trying to finesse details of a workable plan for the Sussexes’ future following the shock announcement by the couple that they are to step back from senior roles in the royal family and work towards financial self-sufficiency. Sources say they are hopeful of a quick resolution.

One key issue will be who finances their personal security when in Canada. The couple deleted a line from their new website sussexroyal.com claiming they have the status of “internationally protected people” and are therefore entitled to personal security under a Home Office mandate. It is understood the line was removed because it breached longstanding official policy not to comment on royal security matters.

The Duke and Duchess of Cambridge spent the day visiting community projects in Bradford, west Yorkshire.