The late Princess Diana's brother believes she had "very severe ADHD".

Earl Charles Spencer was diagnosed with the neurodivergent condition, which can cause trouble focusing, hyperactivity and impulsive behaviour, as an adult and he can see parallels between his own behaviour and that of his late sister.

The earl is also convinced Diana, who died in a car accident in 1997, had the learning disorder dyslexia because she struggled at school.

He told People magazine: "Look, Diana was born in 1961, and certainly the private school system in England was not tuned into children's needs as they are today.

"I think Diana suffered from dyslexia. She found spelling and word orders and letter orders quite tricky.

"I was only found out to have very severe ADHD late in life, and I know it's exactly what Diana had. It's so similar.

"But Diana was as extreme. So I've got really upset when people have tried to say, because Diana could get very hurt and emotional, they would say, 'Oh, maybe she had bipolar or borderline personality' and all these. Who knows? I know it's not those, by the way, but these people throw out these theories. I think it's more humdrum in a way. I think it was ADHD, and rejection for people with ADHD goes very, very deep."

Charles' new book, Swan Song: Diana, My Sister, details the earl's memories of his sister abruptly freezing out members of her inner circle, and he believes she behaved that way because of undiagnosed ADHD.

He said: "This isn't a criticism. It's a recognition that she was struggling with a condition that was totally undiagnosed, and she could turn on a pin. If she felt a friend had let her down, that was a really big deal and it was difficult to come back from that.

"I'm very straight in the book. There wasn't a single member of my family who Diana didn't fall out with and turn her back on for some time.

"But I suppose as family, we weren't going anywhere, so we came back into favour or whatever. But understanding the ADHD aspect of that is really helpful. And also the extraordinary pressure she was under...

"People said, 'Oh, she was paranoid.' No, she wasn't. She was sensitive to rejection, and she felt very let down by people.

"And because of her ADHD, because of her personality and her character, she felt these things very deeply."

The earl worried Diana used to "give" too much of herself to other people, leaving her exhausted.

He added: "And I think particularly so because she was giving so much of herself in her public life.

"I used to feel this extraordinary thing about Diana, that it was almost like people had no idea how much they were taking from her. And I'm talking about innocent members of the public, who adored her, but she felt the need to give. I think it made her feel better about life, but it was draining.

"When she was at home alone in Kensington Palace, she'd often go to bed by 9:00 because she was just done. And then the next day she'd go and do a public event and be the life and soul of the day and make these people's... Give them life memories. This is not done without cost to the individual."