As the Duke and Duchess of Sussex prepare to fly home from their South African tour, the tabloid newspaper accused of unlawfully publishing a private letter from Meghan to her estranged father has said it will defend itself "vigorously.

The duchess is suing the Mail on Sunday over the hand-written letter to Thomas Markle. She claims that it was part of a campaign by parent company Associated Newspapers to write "false and deliberately derogatory stories about her, as well as her husband".

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A wave of positive coverage has engulfed the pair since they landed in Cape Town on September 24, forensically documenting their meetings with presidents, charity workers, young people and Nelson Mandela's widow, Graca Machel over the ten day tour.

Harry also travelled to Botswana, Malawi and Angola, where he visited the same minefield his mother walked through 22 years ago.

Meghan is an ardent campaigner for women's rights, drawing attention to initiatives that promote women's education, health and entrepreneurship during the tour.

Making his royal tour debut, four-month-old Archie Harrison's introduction to Archbishop Desmond Tutu was treated to particularly exuberant reporting.

On Tuesday, Harry published an impassioned plea to the press to cease and desist their "relentless propaganda" and comparing the tabloids' treatment of his wife to that of his mother.

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

Princess Diana was killed in a car crash in 1997 after being hounded by the paparazzi in Paris. During his eulogy of his sister, Earl Spencer described her as "the most hunted person of the modern age."

In the powerful personal statement, the duke said his "deepest fear is history repeating itself".

He wrote: "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."

Prince Harry went on to say "positive" coverage of the couple's tour had exposed the "double standards" of "this specific press pack that has vilified [Meghan] almost daily for the past nine months".

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Meghan has been a target for the tabloids since it was revealed in 2016 that she and the prince were dating. It prompted him to issue a stern statement to the press about its coverage of the former TV actress, after "a line had been crossed" citing "racial undertones" in their reporting.

In 2012, Prince William and Kate Middleton sued a French magazine for publishing topless photographs of her during a holiday at a private villa. In 2017, a French court ruled Kate's privacy had been invaded and the publisher ordered to pay out 100,000 Euros in damages.

Critical coverage of Harry and Meghan has included their use of private jets to their decision not to allow media coverage of Archie's christening. Meghan's father has been fodder for the tabloids, who have mined their fractious relationship for material including running staged photos of him ahead of the royal wedding.

The duke has been criticised for his approach in trying to protect his wife. Journalist and former Daily Mirror editor Roy Greenslade said: "Is he taking a sledgehammer to crack a nut here? I think he may well find that this is counter-productive." Journalist and biographer Penny Junor called the statement an "over-emotional and somewhat ill-advised outburst ", while others have questioned the timing of the case.

The Mail on Sunday said in a statement: "The Mail on Sunday stands by the story it published and will be defending this case vigorously."