According to the law firm Schillings – which is representing the Duchess of Sussex – she has filed a High Court claim against the paper and its parent company, Associated Newspapers.
The case concerns the alleged misuse of private information, infringement of copyright and breach of the Data Protection Act 2018.
A spokesperson for the firm claimed the “intrusive” publication of the letter was part of the media group’s campaign to write “false and deliberately derogatory stories about her, as well as her husband”.
In a statement on the Duke and Duchess of Sussex’s official website on Tuesday, Prince Harry said the couple had been driven to take legal action following “relentless propaganda” about Markle.
“Unfortunately, my wife has become one of the latest victims of a British tabloid press that wages campaigns against individuals with no thought to the consequences – a ruthless campaign that has escalated over the past year, throughout her pregnancy and while raising our newborn son,” he wrote.
Prince Harry continued: “There comes a point when the only thing to do is to stand up to this behaviour, because it destroys people and destroys lives. Put simply, it is bullying, which scares and silences people. We all know this isn’t acceptable, at any level.
“We won’t and can’t believe in a world where there is no accountability for this,” he said, saying it was his “deepest fear” that history would repeat itself.
“I’ve seen what happens when someone I love is commoditised to the point that they are no longer treated or seen as a real person. I lost my mother and now I watch my wife falling victim to the same powerful forces.”
The legal action is being funded privately by Harry and Meghan. Depending on the court ruling, proceeds from any damages will be donated to an anti-bullying charity.
The Schillings spokesperson added: “Given the refusal of Associated Newspapers to resolve this issue satisfactorily, we have issued proceedings to redress this breach of privacy, infringement of copyright and the aforementioned media agenda”.
A Mail on Sunday spokesman said: “The Mail on Sunday stands by the story it published and will be defending this case vigorously.
“Specifically, we categorically deny that the duchess’s letter was edited in any way that changed its meaning.”