Meghan Markle, the Duchess of Sussex, has started legal action against the Mail on Sunday newspaper over an allegation it unlawfully published one of her private letters.
Law firm Schillings, representing the duchess, said she had filed a High Court claim against the paper and its parent company Associated Newspapers over the alleged misuse of private information, infringement of copyright and breach of the Data Protection Act 2018.
A spokeswoman 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 lengthy and highly personal statement published on the Duke and Duchess of Sussex's official website, Prince Harry said the royal couple had been driven to take legal action after "painful" impact of "relentless propaganda" against his wife from the British tabloid press.
Referencing press coverage of his mother Princess Diana, the duke said his "deepest fear is history repeating itself".
He wrote: "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 proceedings in the Chancery Division of the High Court are being privately funded by Harry and Meghan.
Depending on the court ruling, proceeds from any damages will be donated to an anti-bullying charity.
The Schillings spokeswoman 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."
In his statement, the duke said he and the duchess believed in "media freedom and objective, truthful reporting" as a "cornerstone of democracy".
Addressing readers, he added: "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.
"There is a human cost to this relentless propaganda, specifically when it is knowingly false and malicious, and though we have continued to put on a brave face - as so many of you can relate to - I cannot begin to describe how painful it has been.
"Because in today's digital age, press fabrications are repurposed as truth across the globe. One day's coverage is no longer tomorrow's chip-paper."
Prince Harry said the couple, who are currently on a royal tour of Southern Africa, had been unable to correct "continual misrepresentations" in the press.
He said "positive" coverage of the tour exposed "the double standards of this specific press pack that has vilified her almost daily for the past nine months; they have been able to create lie after lie at her expense simply because she has not been visible while on maternity leave.
"She is the same woman she was a year ago on our wedding day, just as she is the same woman you've seen on this Africa tour."
The duke added: "For these select media this is a game, and one that we have been unwilling to play from the start.
"I have been a silent witness to her private suffering for too long. To stand back and do nothing would be contrary to everything we believe in."
He explained that the alleged unlawful publication of the private letter was done in "an intentionally destructive manner" to "manipulate" readers.
The duke claimed readers were misled by "strategically omitting select paragraphs, specific sentences, and even singular words to mask the lies they had perpetuated for over a year".
He added: "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."
The Mail on Sunday has been approached for comment.