Betrayal: Power, Deceit and the Fight for the Future of the Royal Family, parts of which have been serialised by The Times magazine, features a number of accusations against the Sussexes, their relationship with the Prince and Princess of Wales and the Invictus Games. Harry and Meghan’s statement, which they released last weekend, read: “Mr Bower’s commentary has long crossed the line from criticism into fixation. This is someone who has publicly stated, ‘the monarchy in fact depends on actually obliterating the Sussexes from our state of life,’ language that speaks for itself.

“He has made a career out of constructing ever more elaborate theories about people he does not know and has never met. Those interested in facts will look elsewhere; those seeking deranged conspiracy and melodrama know exactly where to find him.”

Now, a PR expert said the lengthy, “emotional” statement was a “very common mistake” and claimed the couple should have either kept quiet or released a very short statement instead.

Renae Smith, founder and director of Atticism, told the Daily Express: “That is one of the most common mistakes I see in reputational crises.

“Clients want to engage with each accusation, dismantle the author, and defend themselves point by point. But in doing so, they often give the criticism more life and, strangely, more credibility.

“The very act of engaging can make the audience feel there is substance there.”

She added that the Sussex statement showed that “Bower hit a nerve,” but explained: “Not necessarily because every claim is true word for word, but because it is clearly close enough to the public narrative around them to sting.”

Ms Smith said that she would have advised the couple to “either say nothing at all, or issue one very short line declining to comment”.

She concluded: “I don’t think the statement benefited them. I think it amplified the book, reinforced the sense that they are thin-skinned, and added fuel to a narrative that was already damaging enough on its own.”