Prince Harry refers to Prince William as his "beloved brother and archnemesis" in his new memoir.
In a teaser trailer for an upcoming sit-down interview with Good Morning America's Michael Strahan, the British royal was asked about the "strong words" he used to describe his relationship with his sibling in the book Spare.
"There has always been this competition between us. It really plays into, or is played by, the heir/spare," he said.
In the memoir, Harry reflects on his childhood, the death of his mother Princess Diana in 1997, and the tension he has with William and their father King Charles III following his and his wife Meghan, Duchess of Sussex's decision to step down as senior members of the royal family in early 2020 and move to the U.S.
"Ultimately, I don't think we can ever have peace with my family unless the truth is out there," the 38-year-old continued.
The full interview is set to air on 9 January, a day before Spare will hit shelves.
Meanwhile, other excerpts from the book have dropped online.
According to editors at Us Weekly, in one chapter Harry recalls mentally bidding "goodbye" to William after his wedding to Catherine, Princess of Wales in 2011.
In addition, the father-of-two also apparently begged Charles not to marry Camilla, Queen Consort in 2005.
"Despite Willy and me urging him not to, Pa was going ahead. We pumped his hand, wished him well. No hard feelings," according to an excerpt obtained by Page Six. "We recognised that he was finally going to be with the woman he loved, the woman he'd always loved."
Representatives for Buckingham Palace have not yet commented on the claims.