The Duchess of Sussex's father is prepared to testify against her over a claim that a tabloid newspaper unlawfully published one of her private letters to him.
Legal documents seen by The Telegraph have confirmed that Thomas Markle's evidence will form part of the Mail on Sunday's defence against the Duchess's legal action for breach of privacy, copyright and data protection.
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The court papers disclose text messages sent from Mr Markle to his daughter and lay bare the deteriorating relationship between the pair at the time of her wedding to Prince Harry.
The papers were filed at the High Court on Tuesday, a day after the Queen released a statement confirming the couple's split from the Royal family.
The revelation comes after a torrid week for the monarchy, which was prompted by last Wednesday's "personal message" from the Duke and Duchess about their intentions, which left the Queen "hurt" and "disappointed" and culminated in a family summit at Sandringham
The Royal family is working out a deal that will enable the Sussexes to adopt a new role that could give them financial independence and allow them to spend more time in Canada.
The Duchess was on Tuesday night seen for the first time since returning to Canada, boarding a seaplane on Vancouver Island without her son, Archie.
If the Duchess's case against the Mail on Sunday goes to trial, it is likely Mr Markle will testify against his daughter and the Duchess will be forced to give evidence against him. Submitted by the Mail on Sunday's parent company, Associated Newspapers, the 44-page defence accuses the "self-promoting" Duchess of "knowingly" making public the contents of the letter to paint her in a more flattering light
The documents, seemingly written with Mr Markle's co-operation, justify publication of excerpts from the letter and his response to it last February, insisting it was "necessary for the sake of truth, fairness, and Mr Markle's reputation, and so that the public should not be misled". It adds: "The Claimant's privacy rights do not extend to silencing her father."
Last October, the Duke of Sussex said he and his wife had been forced to take action against "relentless propaganda" from Associated Newspapers' publications in an emotive attack on what he described as a "ruthless campaign" against his wife. Accusing the tabloid media of "bullying" behaviour that "destroys people and destroys lives", he evoked memories of his mother, Diana, Princess of Wales, saying his "deepest fear is history repeating itself".
The Mail on Sunday published the excerpts, along with an interview with Mr Markle, four days after the American magazine People ran articles said to have been based on anonymous interviews with five of the Duchess's closest friends standing up against the "global bullying" she had faced.
As well as confirming the existence of the letter, the unnamed "sisterhood" told People that Mr Markle had never contacted his daughter, claiming that the Duchess was "calling, texting, even up to the night before the wedding" on May 19, 2018 after he had to pull out following an emergency heart procedure.
But according to the court documents, the last message Mr Markle received was a text allegedly sent on May 17 "admonishing him for talking to the press, telling him to stop and accusing him of causing hurt to his daughter". It was purportedly signed: "Love M and H." The defence also claims the couple "did not ask how he was or how the surgery had gone".
The document includes details as to how Mr Markle felt "hung out to dry" and that no one came to see him ahead of the wedding, whereas the Duchess's mother, Doria Ragland, had been personally informed of the royal engagement by two British embassy officials who visited her Los Angeles home.
Questioning claims that the Duchess called him 20 times when he was in hospital, the defence alleges Mr Markle did not receive "any cards or well wishes" and even contradicts the Duchess's claim that she funded herself through university, saying: "Mr Markle had supported the Claimant throughout her childhood and youth. He had paid her private school fees. He had paid all her college tuition, and after she left Northwestern University he continued to pay off her student loans, even after she had landed a well-paid role in Suits."
The court papers state: "Following the publication of the People interview and reports of the People interview, neither the existence nor the contents of the Letter were confidential.
"Mr Markle was also entitled publicly to correct the false and damaging (to him) information that had been given about his conduct in the People interview, and to have as much of the Letter and its contents published as was necessary for that purpose."
The defence also alleges that Mr Markle has not heard from his daughter since August 2018, when she sent the "immaculately copied out" and "self-congratulatory" letter.
Pointing out that it was written "in her own elaborate handwriting" with "no crossings-out of amendments", the newspaper group claims "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". It points out that the Duchess "chose to stay silent" on the question of whether she authorised her friends to talk to People.
The magazine published a story quoting a "family friend" on Tuesday who said the couple felt they had no choice but to manage things the way they did.
The proceedings in the Chancery Division of the High Court are being funded privately by the Sussexes, with any proceeds from damages awarded to be donated to an anti-bullying charity. Yet legal experts said the couple may have already spent about £350,000 on the action so far. Sources close to them insist they are "determined to have their day in court" and will take the case "all the way".
A Mail on Sunday spokesman said it stood by its previous statement and would defend the case "vigorously".