In a highly personal and scathing statement, Harry said some newspapers had “vilified her almost daily for the past nine months” and claimed they had published “lie after lie” at Meghan’s expense simply because she was out of public view on maternity leave.
Referencing his mother Diana, Princess of Wales, who was a tabloid newspaper staple and died in a Paris car crash while being pursued by the paparazzi, the duke said: “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.”
Harry said about his wife: “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.”
The couple are coming to the end of a 10-day tour of southern Africa with son Archie which has received favourable coverage, but the duke said these positive publications exposes the “double standards of this specific press pack”.
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 Mail on Sunday editor, Ted Verity, refused to comment on the legal action taken against the newspaper when approached by ITV News, but a spokesperson said the paper would be "defending this case vigorously."
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.”
It is understood the couple did not take legal action lightly, but the timing of the announcement will overshadow the final day of their Africa tour on Wednesday.
The couple were all smiles on their final day of the 10-day tour, where they arrived in a Johannesburg township to learn about a project, which aims to tackle the critical issue of youth unemployment.
Prince Harry said in a speech: "We will firmly stand up for what we believe.
"We are fortunate enough to have a position that gives us amazing opportunities, and we will do everything that we can to play our part in building a better world.
"We will always seek to challenge injustice and to speak out for those who may feel unheard."
While Meghan thanked the people of Africa for welcoming them so "warmly, strongly, beautifully."
She added: "There's so much talent here, there's so much ingenuity here, there's so much promise here. Given the level of support and resources you need, the potential is astronomical."
In February, the Mail on Sunday published extracts of Meghan’s handwritten letter to her estranged father Thomas Markle.
In the run up to the wedding, Mr Markle was caught up in controversy after he allegedly staged paparazzi photographs of himself and then began commenting regularly to entertainment website TMZ about his contact with his daughter.
Harry explained that the alleged unlawful publication of the private letter was done in “an intentionally destructive manner” to “manipulate” readers.
He 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.”
A spokeswoman for law firm Schillings claimed the “intrusive” publication of the letter was part of Associated Newspaper’s campaign to write “false and deliberately derogatory stories about” Meghan, “as well as her husband”.
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.”