Meghan, Duchess of Sussex has launched legal action against the Mail on Sunday over a claim that it unlawfully published a private letter from her to her father Thomas Markle.
In a statement, Prince Harry and Meghan said they were taking action against "the long and disturbing pattern of behaviour by British tabloid media."
Prince Harry spoke of his wife's "private suffering" and said his "deepest fear" was history repeating 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."
Prince Harry said the "double standards" of the tabloids was exposed by the favourable coverage of the couple's tour of Africa.
"The positive coverage of the past week from these same publications exposes 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 claim against Associated Newspapers, owner of the Mail on Sunday, is over the misuse of private information, infringement of copyright and breach of the Data Protection Act 2018.
A spokesperson for Schillings, the firm representing The Duchess of Sussex, said:
"We have initiated legal proceedings against the Mail on Sunday, and its parent company Associated Newspapers, over the intrusive and unlawful publication of a private letter written by the Duchess of Sussex, which is part of a campaign by this media group to publish false and deliberately derogatory stories about her, as well as her husband.
"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".
The case is being privately funded by The Duke and Duchess of Sussex. Pending a Court ruling, proceeds from any damages will be donated to an anti-bullying charity.
- More to come