The Duchess of Sussex’s legal action will be the “privacy case of the century” if it goes to trial, a media law expert has said.

Meghan is suing the Mail on Sunday after allegations it unlawfully published a letter she wrote to her father.

The Duke of Sussex launched an unprecedented attack against the British tabloid press for a “ruthless campaign” against his wife, as the legal action was announced.

Meghan and Harry
Meghan with Harry during their tour to southern Africa (Chris Jackson/PA)

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.

David Banks, a journalist and expert in media law, told the PA news agency a trial would go global, could cost Harry and Meghan millions, and could actually lead to more revelations about the duchess’ relationship with her estranged father Thomas Markle.

If the case is not settled beforehand, the duchess, and the duke as well, could even appear in court to give evidence, just as singer Sir Cliff Richard did when he sued the BBC over its coverage of the police search of his home.

Mr Banks said: “Cliff Richard gave quite telling evidence about the effect this had on him and the intrusive nature of the coverage.

“It would be very powerful if Meghan does or Harry does as well. It is very unusual for royals to do. I can’t think of a case in living memory.”

He added: “If it comes to court, it could be the privacy case of the century.”

Cliff Richard court case
Sir Cliff Richard won his High Court privacy battle against the BBC (Victoria Jones/PA)

In February, the Mail on Sunday published extracts of Meghan’s handwritten letter to Mr 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.

Law firm Schillings, representing the duchess, said on Tuesday that Meghan 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.

Mr Banks said: “This is going to be global if it gets to trial. I cannot think of another privacy case of this magnitude with figures at the centre of it that are the level of the Royal Family.

“There have been plenty of celebrity privacy cases in the past but they are nowhere near as significant and of interest to the public.”

Meghan is accompanied to the altar by the Prince of Wales after her father Thomas Markle pulled out of attending her wedding
Meghan is accompanied to the altar by the Prince of Wales after her father Thomas Markle pulled out of attending her wedding (Jonathan Brady/PA)

Meghan and Harry, who are on the final day of their now-overshadowed 10-day official tour to southern Africa with their baby son Archie, are likely to be braced for a “very uncomfortable” exploration of the duchess’s relationship with her father, Mr Banks added.

“It is one of the drawbacks of a privacy action for the claimant. The very thing that you want to remain private becomes the topic of a hugely publicised High Court legal action,” he said.

“There is the distinct possibility that other aspects of Meghan’s relationship with her father and why the Mail on Sunday felt that was in the public interest to explore in their journalism might be at the heart of this legal action which could make things very uncomfortable and even more revelatory for the duke and duchess.

“But clearly, they’ve taken the action. That must be something they’re prepared for.”

The media watch Harry and Meghan depart after a visit to the Tembisa township in Johannesburg
The media watch Harry and Meghan depart after a visit to the Tembisa township in Johannesburg on Tuesday (Dominic Lipinski/PA)

In the statement published on the duke and duchess’s official website, Harry referenced 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.

“Though this action may not be the safe one, it is the right one,” he said.

“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’s high-profile overseas trip has received favourable coverage, but the duke said these positive publications expose the “double standards of this specific press pack”.

The Duke and Duchess of Sussex with their baby son Archie in South Africa
The Duke and Duchess of Sussex with their baby son Archie in South Africa (Toby Melville/PA)

He 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”.

The case is being privately funded by the Sussexes and any proceeds from damages will be donated to an anti-bullying charity.

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.”

Once a claim is filed with the High Court, the defendant generally acknowledges service of the legal action before issuing a defence.

It is usually many months before a trial takes place and there may be a series of hearings ahead of a trial to deal with preliminary legal issues.

The parties may settle the claim out of court at any stage of the proceedings and, if they do, there is likely to be a statement made in open court which tends to include an apology from the defendant.

The High Court has dealt with a number of high-profile privacy cases in recent years, including Sir Cliff’s action against the BBC and the ongoing litigation over alleged phone hacking by the publishers of the Mirror and the now-defunct News of the World.