Meghan Markle is suing Britain's Mail On Sunday newspaper over the publication of a private letter, her husband Prince Harry has said, warning they had been forced to take action against "relentless propaganda".

In a stinging rebuke of British tabloid media, the Duke of Sussex described his wife as being hounded by the press in the same way as his mother Princess Diana was before her death in a Paris car crash in 1997.

"My deepest fear is history repeating itself," he said in a statement on Tuesday. "I lost my mother and now I watch my wife falling victim to the same powerful forces."

Prince Harry said the couple would take legal action over the contents of private letter, which were "published unlawfully in an intentionally destructive manner".

Addressing newspaper readers, he said the article had "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".

The statement did not reference a specific letter but earlier this year the tabloid published an article about a handwritten letter that Meghan had sent to her estranged father Thomas Markle.

"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," the Duke of Sussex said.

"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," he added.

The statement comes months after George Clooney warned the Duchess of Sussex, who was seven months pregnant at the time, was being "vilified and chased" by the press much like Princess Diana had been.

Britain's famously aggressive press at first welcomed Markle into the royal fold and the mixed-race actress was credited with breathing fresh life into a monarchy sometimes labelled stale and out of touch.

But coverage has turned increasingly critical and tabloids have luxuriated in stories about Markle's fractured American family.