The Duchess of Sussex is to sue the Mail on Sunday over allegations that it unlawfully published one of her private letters.

It comes as the Duke of Sussex issued an extraordinary statement saying he fears his wife is "falling victim" to press intrusion as his mother did.

He condemned what he called a “ruthless campaign” against the Duchess, saying: “I lost my mother and now I watch my wife falling victim to the same powerful forces.”.

In a statement, the Duke of Sussex said he and Meghan were forced to take action against "relentless propaganda".

The Duke, who is currently in South Africa with his wife and son, accused the tabloid media of “bullying” behaviour which “destroys people and destroys lives” with "continual misrepresentations". 

The Herald: The earrings the Duchess of Sussex wore on a visit to Cheshire were a gift from the Queen (Eddie Mulholland/Daily Telegraph/PA)

Referring to his late mother Diana, Princess of Wales, he said his "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," he said.

In a written statement issued on a dedicated website, the Duke said he “cannot begin to describe how painful” it has been to read “relentless” negative stories about the Duchess, emphasising that she is a new mother.

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."

Meghan launched legal action against the newspaper over an allegation it unlawfully published the letter to her father.

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.

The court proceedings are being funded privately by the couple and any proceeds will be donated to an anti-bullying charity.

The full statement from Prince Harry

As a couple, we believe in media freedom and objective, truthful reporting. We regard it as a cornerstone of democracy and in the current state of the world – on every level – we have never needed responsible media more.

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.

Up to now, we have been unable to correct the continual misrepresentations - something that these select media outlets have been aware of and have therefore exploited on a daily and sometimes hourly basis.

It is for this reason we are taking legal action, a process that has been many months in the making. 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.

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.

This particular legal action hinges on one incident in a long and disturbing pattern of behaviour by British tabloid media. The contents of a private letter were published unlawfully in an intentionally destructive manner to manipulate you, the reader, and further the divisive agenda of the media group in question. In addition to their unlawful publication of this private document, they purposely misled you by strategically omitting select paragraphs, specific sentences, and even singular words to mask the lies they had perpetuated for over a year.

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.

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.

We thank you, the public, for your continued support. It is hugely appreciated. Although it may not seem like it, we really need it.