Prince Charles used to joke “Who knows if I’m even your real father?” to his son amid rumours that Harry was James Hewitt’s lovechild, the new book claims.

Harry said that his dad used to joke about an escaped Broadmoor patient he once met who claimed to be the Prince of Wales. Charles would say the patient was so convinced of his identity that it posed “big questions”.

Charles would say, “Which could say which of us was insane. Who knows if I’m really the Prince of Wales? Who knows if I’m even your real father?”

Harry added, “He’d laugh and laugh, though it was a remarkably unfunny joke, given the rumour circulating just then that my actual father was one of Mummy’s former lovers: Major James Hewitt.”

Prince Harry addressed the long-running and scurrilous rumours that he was the secret love child of Hewitt and his mother Diana. Harry claims cavalry officer Hewitt met his mother after he was born. And he said that his father Charles never spoke about the rumours, adding, “If Pa had any thoughts about Major Hewitt, he kept them to himself.”

But he claimed the rumours persisted out of “sadism” and because of Major Hewitt’s ginger hair. He added, “Readers were delighted by the idea that the younger child of Prince Charles wasn’t the child of Prince Charles. Maybe it made them feel better about their lives that a young prince’s life was laughable.”

• A Metro report