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LONDON — The battle between Meghan, the Duchess of Sussex, and the United Kingdom's Mail on Sunday took a surprising turn Tuesday after legal documents submitted by the newspaper suggested that her father could testify against her.

The paper and its parent company, Associated Newspapers, are accused of unlawfully publishing a letter from Meghan to Thomas Markle in February.

The legal documents, seen by NBC News and filed to the High Court of England and Wales, suggest that The Mail on Sunday will use evidence provided by Markle to back up its case.

Its sister paper, the Daily Mail, also wrote on its front page Wednesday that Markle was “set to be a star witness” in the case and “was prepared to give evidence against his own daughter.”

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 legal documents claim.

His daughter from his first marriage, Samantha Markle, later told the BBC: "If he is called, he will come."

The legal documents also say that there is a "huge and legitimate" public interest in the "personal relationships" of members of the royal family.

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Some of these members “generate and rely on publicity about themselves and their lives in order to maintain the privileged positions they hold and to promote themselves,” they add.

The documents are a response to claims filed on behalf of the duchess in October. They allege that the newspaper misused private information, infringed on copyright and breached the U.K.’s Data Protection Act when it published parts of the letter.

A spokeswoman for Meghan's lawyers claimed in a statement released at the time that the "intrusive publication” was part of Associated Newspapers' campaign to write "false and deliberately derogatory stories about" her, "as well as her husband."

Her lawyers declined to comment Wednesday.

Prince Harry also claimed the alleged unlawful publication of the private letter was done in "an intentionally destructive manner" to "manipulate" readers.

The Mail on Sunday denied editing the letter to change its meaning and said it would stand by the story.

The legal documents also argue that the publication of the letter was in response to a "one-sided" article in People magazine in February, which featured interviews with five of Meghan’s unnamed “close friends.”

The article had referenced the letter, the documents say, meaning its existence was already in the public domain. Meghan had not complained about either the article or the mention of the letter, they add.

The lawyers go on to claim that Meghan’s "immaculate" handwriting is proof she intended for the letter to be published.

"The letter appears to have been immaculately copied out by the claimant in her own elaborate handwriting from a previous draft,” the documents state. "There are no crossings-out or amendments as there usually are with a spontaneous draft.

"It is to be inferred also from the care the claimant took over the presentation of the letter that she anticipated it being disclosed to and read by third parties."

The claims come just over 24 hours after Queen Elizabeth II agreed Harry and Meghan could step back as senior royals and begin a "new life" as an "independent" family.

Buckingham Palace also confirmed the Sussexes would begin a "transition period" in which they would split their time between the U.K. and Canada.

The move has raised questions over the costs of the couple's security during their time in North America and who would foot the bill.

Canadian Prime Minister Justin Trudeau said the costs, and what role the couple will play in Canadian life, would be the subject of future discussions.