Prince Harry has spoken about the "powerful forces" that led to his mother's death as he and Meghan announced they are suing a Sunday newspaper.
In a strongly worded statement, the Duke of Sussex said his wife had become a victim of the British tabloid press and denounced what he called their "ruthless campaign".
He said he feared history repeating itself as he watched his "wife falling victim to the same powerful forces" which plagued his mother Diana's life.
The legal action relates to the publication in The Mail on Sunday of a private letter which Harry and Meghan claim was published in an "intentionally destructive manner" to "manipulate" and to "further the divisive agenda" of the newspaper.
Law firm Schillings, representing the duchess, said it has filed a High Court claim against The Mail On Sunday and its parent company Associated Newspapers over the alleged misuse of private information, infringement of copyright and a breach of the Data Protection Act.
Harry claims readers were misled by the newspaper "strategically omitting select paragraphs, specific sentences, and even singular words to mask the lies they had perpetuated for over a year".
A spokesman for the newspaper 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."
The duke's statement continued: "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."
While defending press freedom, he said he and Meghan had been "unable to correct the continual misrepresentations" and criticised "select media outlets" for exploiting this "on a daily and sometimes hourly basis".
Prince Harry also criticised the "double standards" of the "specific press pack" which has run positive coverage since the couple has been in Africa on a royal tour.
He added: "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."
He called it a "game" he and his wife were unwilling to play.
He said: "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."
His statement added: "Though this action may not be the safe one, it is the right one.
"Because my deepest fear is 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 also thanked the public for the ongoing support he and his wife had received and said "we really need it".
Sky's royal correspondent Rhiannon Mills said their words could not have been stronger, and indicated that the couple may be feeling fragile and under attack.
The news comes as the royal couple come to the end of their tour in Africa, their first as a family with baby Archie.
The Sussexes will fund the legal case privately and donate any proceeds in the event that they are successful to an anti-bullying charity.
A spokesman for Schillings said: "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 Mail On Sunday has run a series of stories with Meghan's father, Thomas Markle, including what turned out to be staged paparazzi photographs of him in Mexico, shortly before her wedding to Harry.
The Guardian reported in February that the Sussexes were threatening legal action, as copyright remains with the author of the letter even if it is in the possession of the recipient.
It reported at the time that Meghan was unhappy about the decision to publish a letter she had written to her father about his multiple media interviews.
Mr Markle was due to fly over for their wedding but did not. He had been working with TMZ, a US-based showbiz website, at the time.
Harry and Meghan are not the first royals to sue sections of the media. Prince William and his wife Kate were awarded damages in 2017 after a French magazine posted long lens photos of the Duchess of Cambridge topless while they were on holiday.
It is also not the first time Prince Harry has made such a strong statement to the press about Meghan. When the couple were dating in 2016, he issued a statement about the "racial undertones of comment pieces" related to her as their relationship came into the spotlight.
He also mentioned "the outright sexism and racism of social media trolls" amid concerns for her safety and that of her mother, Doria Ragland.
Analysis by Sky News and Storyful earlier this year uncovered a wealth of racist and conspiracy-led attacks focused on the Duchess of Sussex.
One of the most common theories spreading online was that she was faking her pregnancy.
Compared with other members of the Royal Family, Meghan suffers a higher rate of online abuse. Of the top 30 Twitter accounts mentioning her name, 18 share abusive posts about her, while top accounts mentioning Kate Middleton were also found to share negative posts about Meghan.
She has been the subject of ongoing speculation about her position in the royal household, with reports of "tantrums", and her father claiming she is controlling, and "shunned and ghosted" him.
Mr Markle even suggested the Queen should get involved to repair their apparently broken relationship.