An alleged royal affair is addressed in Endgame: Inside the Royal Family and the Monarchy's Fight for Survival. The book which is penned by royal expert Omid Scobie, discusses the rumor that Prince William had an affair with his wife, Kate Middleton's, best friend Rose Hanbury.

The gossip spread like wildfire until it slowly got out of the rumor mill once no one addressed it officially. The rumor began in 2019 via a U.K. gossip newspaper. The royals are always under a watchful eye but after the rumor took place, eagle-eyed fans noticed that Middleton was seemingly being cold towards William at an outing which made onlookers believe the rumor. Scobie told Entertainment Tonight that even addressing the rumor was a touchy subject in the book.

“I was very careful in the book to really focus on this, as the allegations against William, Kate's and their fallout with Rose Hanbury,” he said. “For legal reasons there are so many things that one can't go into but I thought it was really important, even if a rumor is a rumor. And I really don't see proof that there is more to this than just a tittle-tattle, you know.”

However, he doesn't believe that the rumor is true.

“Unfortunately, if a rumor's left to kind of do its own thing it can run 20 laps around the world before you even think about what, how you want to kind of address it,” the royal expert and author tells ET's Kevin Frazier in an exclusive interview. “They never addressed it, so those rumors will never go away even though there's no truth to suggest that they are true.”

While he doesn't believe that the rumor is true he did weigh in on what the impact could have been, how it would have been dealt with and what personal impact it would have been on Prince William.

“I thought it was really interesting to analyze how the palace dealt with that,” the author said. “The worst case scenarios that they feared would happen, you know, just those rumors themselves were going to have enough impact, negatively, on William's reputation. We still see them [the rumors] trend on Twitter on a regular basis … that's something that's incredibly damaging, I think, for William. It probably looks even worse, actually, that there was a kind of willingness to throw Harry under the bus simply to make these things disappear.”

Endgame: Inside the Royal Family and the Monarchy's Fight for Survival is out now.