Royal biographer Tina Brown claimed that being the "spare to the heir" made Prince Harry feel marginalised and it eventually caused tension in his relationship with Prince William.

The author of "The Palace Papers: Inside the House of Windsor, the Truth and the Turmoil" contradicted claims that Meghan Markle pushed the brothers apart. She called it "unfair" that the former "Suits" actress gets the blame because the Duke of Sussex is at fault too.

"There were real cracks that were happening which surprised me actually. There's common wisdom that Meghan broke up the brothers, that's actually unfair. She reinforced it if you like," Brown said during an interview on ITV's "Lorraine" on Wednesday.

She said that the brothers "had begun to have a growing distance between them because Harry was so happy in the army." He served ten years in the army with two tours in Afghanistan. She noted, "It was a great success, his whole military career."

However, the Duke of Sussex "felt a bit lost" when he left the army. Brown claimed that "he kind of rattled around while his brother was now set on the path for kingship." Prince William, the next heir to the throne after Prince Charles, "was now being groomed to be king, his destiny was clear, his path was clear."

But for Prince Harry, the author said that "at that point, the memo really hit that he was number two and he was going to be treated like number two." This allegedly made him "feel marginalised" and so it was a "superstar moment" for him when he launched the Invictus Games. It was close to his heart and he felt significant.

Brown explained, "He realised, 'I can do this myself, I have my own power base.' And it was at that point that he met Meghan and she reinforced that sense for him, that there was a bigger, wider world out there than being the number two brother who had to wait in line for the royal assignments."

This is not the first time that Brown defended Meghan Markle from claims that she broke Prince Harry and Prince William's close bond. She said the Duke of Sussex had always wanted out of his royal life and his wife only helped make that happen.

Meghan Markle, Prince Harry and Prince William
Prince Harry last year revealed that he had grown apart from his older brother William Photo: AFP / Tolga AKMEN