Meghan Markle is a silent observer, preparing her new move

The royal biographer, known for his critical remarks about the Sussexes, has suggested the Duchess, 42, is about to “drop another bombshell” about the royal family.

Mr Bower, known for his outspoken views on the couple, spoke to Sun Royal Editor Matt Wilkinson on the Royal Exclusive Show.

In remarks that were particularly scathing even by his own standards of criticism of the couple, he said: “The outlook for the Sussexes is pretty bleak; I think they are in a sustained decline.”

“But every time they need money, every time they need publicity to maintain their reputation, they will drop another bombshell. We are still waiting for Meghan’s autobiography, which I am sure she is writing, she is a good writer, and it will be filled with vitriol and lies.

“She won’t tell the truth because she wants a sensational headline. That’s the ultimate weapon she can deploy.”

It comes as Mr Bower made even more shocking claims that Meghan is a “toxic agent” and “doesn’t seem to care” about her father Thomas Markle.

Despite the drama caused by Harry and Meghan’s vile attacks in Spare, their Netflix documentary and various explosive interviews, the couple have remained silent for some time.

But when asked if the Sussexes had deliberately “tamed” their attacks, Mr Bower claimed Meghan appeared to be plotting something bigger.

“I think she’s let Harry take charge, she’s saving time for her autobiography, it’s much better for her to be seen as the silent duchess, the silent weapon in reserve,” he said.

“Harry said some pretty unfortunate things.”

The announcement comes after the Royal Family was shocked by a prostate cancer diagnosis. In February, Her Majesty bravely revealed that she had been diagnosed with the condition after going to the London Clinic for prostate surgery – the Palace had previously confirmed it was not prostate cancer.

He has since returned to his royal public duties with his usual energy and famous “workaholic approach”.

On Wednesday he delivered the King’s Speech and last month he was in high spirits at his official birthday parade, Trooping the Colour.

Meanwhile, Princess Kate has remained largely out of the public eye since being admitted to the same hospital for major abdominal surgery in January. The brave royal then revealed she was undergoing preventative chemotherapy in March.