Haskell, 37, told OK! magazine: "The royal family is a dysfunctional family, pretending they're ordained by God and they're not. They're literally like every family in the world."
Watch: James Haskell defends Prince Harry for publishing tell-all book
The former England rugby player - who has said he is still in touch with the prince since he moved to Los Angeles - defended Prince Harry's right to publicly voice his family disagreements in his autobiography.
Haskell said: "Everybody's entitled to an opportunity to say what they want, and when someone feels like they and their partner have been oppressed, lied about, contradicted and put into a corner, I think it's very important [to speak out]."
He recently appeared on Good Morning Britain to talk about his own new wellbeing book where he told his father-in-law Madeley he felt it was important that Prince Harry had been allowed to tell his own story.
He said: "If Emily Pankhurst had never done what she done, women wouldn't have achieved what they've achieved, if Rosa Parks hadn't said I'm not going to get up, certain things wouldn't have happened.
"I'm not likening Harry to them, but I'm saying in respect of standing up for something, you can't let people keep walking over you."
Spare has become the fastest-selling non-fiction book ever, with 400,000 copies sold across hardback, ebook and audio formats on its first day of publication.