The royal biographer, known for his critical remarks about the Sussexes, suggested that the duchess, aged 42, is poised to "reveal another bombshell" concerning the Royal Family.
Mr. Bower, renowned for his blunt views on the couple, spoke with Sun Royal Editor Matt Wilkinson on the Royal Exclusive Show.
In his particularly sharp comments, even by his own standards as a critic of the couple, he remarked, "The outlook for the Sussexes is quite bleak; I believe they are in a sustained decline."
"But whenever they need money, whenever they need publicity to stoke their reputation, they will drop another bomb. We're still waiting for Meghan's own autobiography, which I'm sure she's penning, she's a good writer, and it will be filled with vitriol and filled with lies.
"She will not tell the truth because she wants a sensational headline. That is the ultimate weapon she can deploy."
It comes as Mr Bower made more shocking claimsthat Meghan is an "agent of poison"and"doesn't seem to care"about her fatherThomas Markle.
Despite drama propelled by Harry and Meghan's vile attacks inSpare, theirNetflix documentary, and variousbombshell interviews, the couple have stayed quiet for some time.
But when asked if the Sussexes have deliberately "reigned in" their attacks, Mr Bower claimed Meghan seems to be plotting for something bigger.
"I think she's allowed Harry to make the running, she's reserving herself for her autobiography, that's much better for her to be seen as the silent duchess, the silent weapon in reserve," he alleged.
"Harry has said some rather unfortunate stuff."
It comes after shock cancer diagnoses rocked the Royal Family this year. In February His Majesty bravely revealed he was diagnosed with the disease after going into the London Clinic for prostate surgery - the Palace previously confirmed it was not prostate cancer.
He has since returned to royal public duties with his usual energy and famous "workaholic approach".
On Wednesday, he presentedthe King's Speechand last month was in high spirits at his official birthday paradeTrooping the Colour.
Meanwhile, Princess Kate has been largely out of the public eye since she went into the same hospital formajor abdominal surgeryin January. The courageous royal later shared she was undergoingpreventative chemotherapyin March.