Meghan, Duchess of Sussex has launched legal proceedings against the Mail on Sunday newspaper over what her husband, Prince Harry, referred to as a "ruthless campaign" against his wife.

Key points: Schillings law firm said the lawsuit was over the publication of a private letter

Schillings law firm said the lawsuit was over the publication of a private letter The case is being paid for privately by Prince Harry and Meghan

The case is being paid for privately by Prince Harry and Meghan Princess Diana died in a car crash in Paris in 1997 after being followed through the streets by photographers

In an emotional statement, Prince Harry likened the treatment of his wife to that meted out to his mother, Princess Diana, saying Meghan had "become one of the latest victims of a British tabloid press that wages campaigns against individuals with no thought to the consequences".

Princess Diana died in a car crash in Paris when Harry was 12 years old. ( Reuters: Hugh Peralta )

"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 wrote.

"I lost my mother and now I watch my wife falling victim to the same powerful forces."

Schillings law firm said the lawsuit was over the publication of a private letter in a "campaign by this media group to publish false and deliberately derogatory stories" about the couple.

"We have issued proceedings to redress this breach of privacy, infringement of copyright and the aforementioned media agenda," the law firm said.

Schillings said the Royal couple was paying for the legal action.

A Mail on Sunday spokesman told Britain's Press Association the paper "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 spokesman said.

'My deepest fear is history repeating itself'

Princess Diana became one of the most photographed women on Earth after she married Prince Charles. ( Reuters )

In the opening of his statement, Harry spoke on behalf of himself and his wife, saying they regarded media freedom as "a cornerstone of democracy".

"As a couple, we believe in media freedom and objective, truthful reporting."

But he went on to say the contents of a private letter had been published in an "intentionally destructive" and "manipulative" manner and strategically omitted specific words and sentences.

He said "knowingly false and malicious" propaganda had escalated over the past year, throughout Meghan's pregnancy and while she raised the couple's newborn son, Archie.

"I cannot begin to describe how painful it has been," Harry said, adding: "My deepest fear is history repeating itself."

Prince Harry was 12 years old when his mother died in a car crash in Paris in 1997 after being followed through the streets by photographers.

Princess Diana became one of the most photographed women on Earth after she married Harry's father, Prince Charles.

ABC/wires