And the 74-year-old writer has confirmed his hard-hitting book is well under way, having researched his subject for a year and speaking to friends, foes and associates of Meghan and Prince Harry.

He said: “I’m writing it now. It will tell the truth when it comes out next year.”

Two years ago we were the first newspaper to report that the five-times biographer was planning a book on the Duchess .

Meanwhile, last week it was reported Harry and his American-born wife plan to write four books as part of a lucrative £29million deal with US publishers Penguin Random House – a move Mr Bower described as certain to “damage” the Royal Family” .

Harry has come under fire for collaborating with celebrity ghostwriter JR Moehringer on what his publishers have described as “the definitive account of the experiences, adventures, losses and life lessons that have helped shape him”.

Mr Bower claimed: “The only way that Harry can earn money and justify the advance, and the only thing that the American publishers will be happy with, is if he is very critical of the Royal Family” .

“It will be damaging for them and that is a tragedy.”

The deal means that Harry’s memoir will be released in late 2022, when it is the Queen’s platinum jubilee year.

The book is described by Harry as being a “wholly truthful first-hand account” of his life. But royal aides are worried that it will prove to be a highly biased account of the 36-year-old prince’s experiences and only serve to further heighten tensions with his estranged relatives.

Meghan is also set to pen a “wellness” guide as part of the Random House contract.

In another development, it was revealed that an updated version of Finding Freedom, by Omid Scobie and Carolyn Durand, will be released on the anniversary of Princess Diana’s death, August 31.

The book, which gives a behind-the-scenes account of Meghan and Harry’s life, will now feature a section detailing their heartbreak following the death of Prince Philip in April.

The new section will also recount “Meghan’s emotional healing journey from losing a child to the birth of their daughter” and will detail the couple’s relocation to California, according to publisher HarperCollins.

The couple have long denied personally co-operating with Scobie and Durand. But the writers have boasted of having “unique access and the participation of those closest to the couple”.

The flurry of publications over the next year is sure to captivate readers on both sides of the Atlantic.

In particular, Harry’s memoir is tipped to be a best-seller, with one of his mother’s confidantes saying that it will fulfil Diana’s legacy.

Diana’s private voice and presence coach, Stewart Pearce, said that “absolutely and unquestionably” Diana had been ready to write her own memoir before she died.

She confided to Mr Pearce that “she was receiving offers left, right and centre”.

Mr Pearce asserts that the Duke of Sussex inherited his mother’s bravery and is ready to face the consequences of telling his own story.

‘It will be damaging for Royal Family’