Meghan Markle isn't the first royal woman to sue. Kate Middleton did in 2012. And won.

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Meghan Markle and husband Prince Harry are finally pushing back against a British tabloid, announcing this week they are suing the Mail on Sunday due to their incessant vilification of the duchess.

According to Harry, the lawsuit has been months in the making and is specifically against the newspaper’s publishing of a private letter Meghan wrote to her father, Thomas Markle.

Prince Harry talks about his late mother, Princess Diana, after his engagement to Meghan Markle. Post continues below.

The Duke of Sussex condemned the publication for its “relentless” campaign against his wife.

“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,” Harry wrote in a statement on the Sussex Official website.

“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.”

Harry went on to compare the tabloid pursuit of Meghan to that of his mother.

“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.”

According to the statement, the claim alleged a breach of the Data Protection Act of 2018, which contains provisions that criminalise reckless disclosures of personal information.

But while royal lawsuits are rare, they are not unprecedented.

The Queen has twice sued British paper The Sun for breach of copyright - once in 1988 when it published a stolen photo of Sarah Ferguson and daughter Princess Beatrice, and again when it published leaked text of her 1993 Christmas broadcast.

Every one of Princess Diana's moves were watched by paparazzi, but for all the trouble she had, she only sued once: When the Daily Mirror published photos of her exercising at a gym that were taken with a hidden camera.

All of these lawsuits were settled out of court.

The truth about Meghan Markle's relationship with her dad. Post continues below audio.

Then in 2012, Kate Middleton was snapped sunbathing topless while on holiday at a 640-acre estate in southeastern France. She did not suspect that almost a kilometre away, a paparazzo was lurking with a long-range lens.

He captured the revealing photos and France's Closer magazine published them after teasing: "the future Queen of England such as you have never seen her... and such that you will never see her again!"

The palace was furious and immediately issued a statement that said the "grotesque and totally unjustifiable" invasion of privacy was "reminiscent of the worst excesses of the press during the life of [Princess] Diana," and lawyers were being consulted.

Kate and William filed a lawsuit.

Closer stood its ground, describing the photographs as showing "a beautiful, in love, modern holidaying young couple, in their normal life".

It took five years, but in 2017 a trial got underway.

In a letter, Prince William wrote that the case was "particularly shocking because it reminded us of the harassment that led to the death of my mother, Diana, Princess of Wales".

Closer's lawyer Paul-Albert Iweins argued it was in the public interest to publish the photos.

"It’s of public interest to know that future heirs to the throne have a solid relationship and are getting on well. It’s all part of the royal business," he said.

But the French court disagreed. Five years after they were published, the photos were officially ruled a gross invasion of privacy.

The French magazine and two of its staffers were ordered to pay €190,000 (AU$309,909). Afterwards, a palace spokesperson stated the Duke and Duchess of Cambridge "felt it essential to pursue all legal remedies".

"They wished to make the point strongly that this kind of unjustified intrusion should not happen."

Princess Diana's sons saw how the relentless pursuit by tabloids and paparazzi could end, and they've vowed to stop it from happening again.

The princes and their wives seem determined to ensure that intense invasions of privacy are never again tolerated.