The author says Prince Harry will be sidelined by Prince William and Prince Charles after the Queen's death.
Royal biographer Andrew Morton believes Prince Harry has hit rock bottom amid the rift with his father and it is only the Queen to whom he still holds importance in the UK.
The Queen, author of the newly-released biography, told Express.co.uk: "Harry's main ally within the royal family is the woman Diana called 'Chief Lady', the Queen.
"When she leaves, she will take out her book, which will have a fairly strong attack on her father.
"He's not talking to his brother, who will be the next Prince of Wales, so he'll disappear into the shadows, he'll be on edge - just the way he wants to."
Harry's memoir, which will be released later this year, is "a definitive account of the experiences, adventures, losses and life lessons that have helped shape him".
The book will trace Harry's "a lifetime in the public eye from childhood to the present day".