He explained: “She can laugh when there are either adverse headlines or there’s some crisis or something. She’s just an upbeat person. I could see her in a real crisis, I think she’d be a great asset.”

The author branded the monarch a “doting grandfather” and said he now spends more time at Windsor Castle, which allows him to see the Prince and Princess of Wales’s children, Prince George, Princess Charlotte and Prince Louis, even more often.

The Wales family reside in Adelaide Cottage, which is just a 10-minute walk from the castle. He said: “King Charles is a father who loves his son and a doting grandfather.

“One thing we don’t see, and people weren’t expecting, the King is spending a lot more time at Windsor than anyone expected.”

“He’s often there on Thursdays and Fridays. Obviously, the Waleses are within walking distance of the castle. He’s never really had that sort of proximity to the younger family. Now, there’s more overlap.”