The Duchess of Sussex has launched “privately funded” legal action against a British tabloid newspaper after it published a handwritten letter she had sent to her estranged father.

The decision to begin High Court action against the Mail on Sunday came as Prince Harry went public with a defence of his wife, saying he could no longer be a “silent witness to her private suffering.”

Emphasising his respect for the importance of “objective, truthful reporting”, the Queen’s grandson accused parts of the media of “waging campaigns against individuals with no thought to the consequences” and compared the treatment of Meghan to coverage of his mother, Princess Diana.

The duke wrote that his “deepest fear is history repeating itself” and the time had come for “accountability” over the treatment of his wife. He continued:

There comes a point when the only thing to do is to stand up to this behaviour, because it destroys people and destroys lives. Put simply, it is bullying, which scares and silences people. We all know this isn’t acceptable, at any level. We won’t and can’t believe in a world where there is no accountability for this. Though this action may not be the safe one, it is the right one … 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.

For its part, a spokesperson from the paper 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.”

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 and was a victim of media attention much like Princess Diana had been.

Clooney said the media should “be a little kinder” to her. Clooney’s wife, Amal, attended the duchess’ baby shower and the Clooneys have vacationed with the Royal couple.

The Duke and Duchess of Sussex are currently winding up their 10-day tour of southern Africa. They are travelling with 13 assistants including a “social media officer” and a hairdresser for Markle.

The pair will return with baby Archie and their team of Royal assistants on a long-haul flight to the UK late on Wednesday.