"The Windsors don't do woke… In other words, they don't know how to handle progressive causes,” Robert began, before revealing what he believes made Wills a star player.
As the royal biographer explained, William’s ability to engage with everyone – including royalists and forward-thinking generations – has helped increase his relatability.
“I think William does it better than Meghan and Harry,” Robert told Royal Beat host Kate Thornton.
"This couple are in a way, too extreme,” he added.
"The Royal Family's not got to appeal just to young and trendy people… It's got to mean something to everybody and I think William has found the secret of doing that."
When Kate chimed in: "You find him less preachy?" Robert replied: "Definitely less preachy!"
Despite Charles reportedly been told by the Firm to put an end to his climate change activism, Prince William has now emerged as the new eco advocate on the royal beat.
Speaking in the new ITV documentary, Prince William: A Planet For Us All, the 38-year-old urged people to acknowledge the growing environmental threat to the planet.
“I’ve always loved nature… but fatherhood has given me a new sense of purpose,” William says in the first-look promo.
“Now I’ve got George, Charlotte and now Louis in my life, your outlook on life does change. You want to hand over the wildlife in a much better condition,” he added.
The documentary, which was filmed over the last few years, follows Wills’ travels to Tanzania and Pakistan, where he identified the effects of climate change.