Prince William and Prince Harry open up about Princess Diana's parenting style for the first time

Prince William and Prince Harry are opening up about their mother's parenting style for the first time.

The royals appear in an upcoming documentary for ITV and HBO titled Diana, Our Mother: Her Life and Legacy, in which they discuss the unique way in which Diana raised them.

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"This is the first time that the two of us have ever spoken about her as a mother," 32-year-old Harry shares in a promo clip for the project, calling her "one of the naughtiest parents."

See photos of Princess Diana and her sons:

"She was very informal and really enjoyed the laughter and the fun," 35-year-old William adds. "She understood that there was a real life outside of Palace walls."

"She was our mum. She still is our mum. And of course, as a son I would say this, she was the best mum in the world," Harry said. "She smothered us with love, that's for sure."

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William and Harry recently rededicated their mother's grave in honor of what would have been her 56th birthday.

See more in the video below.