The Duke of Sussex was aged 20 when he was photographed wearing the costume, complete with a swastika on his arm, to a “native and colonial” party hosted by Olympic show jumper Richard Meade.
The move prompted a wave of criticism and he apologised for the “poor choice of costume”, adding that he was “very sorry if I caused any offence or embarrassment to anyone”.
Harry called the decision to wear the costume “probably one of the biggest mistakes of my life” during his and wife Meghan Markle’s six-part Netflix docuseries, which was released in December last year.
On Thursday (5 January), Page Six reported that Harry’s autobiography Spare includes the revelation that the Prince and Princess of Wales “howled” when they saw him dressed up in the costume.
The publication said it had obtained a copy of the book and quoted Harry as writing: “I phoned Willy and Kate, asked what they thought. Nazi uniform, they said.
“They both howled. Worse than Willy’s leotard outfit! Way more ridiculous! Which, again, was the point,” the 38-year-old continues.
William also attended the costume party, dressed in a lion outfit.
The Independent has contacted Kensington Palace for comment.
Harry’s “personal and emotional” memoir has been ghostwritten by Pulitzer Prize winner JR Moehringer and will be released by Penguin Random House on 10 January.
According to The Guardian, which said it had obtained an extract ahead of publication, the book also includes the bombshell claim that William physically attacked Harry over an argument about his marriage to Meghan Markle.
He said William called the Duchess of Sussex “difficult”, “rude” and “abrasive” before he “grabbed my by the collar, ripping my necklace, and he knocked me to the floor”.
The publication also reported Harry as writing that King Charles asked his sons not to “make my final years a misery” following Prince Philip’s funeral last year.
In a new trailer for Harry’s forthcoming interview with ITV’s Tom Bradby, he refused to commit to attending King Charles’s coronation in May.
“The ball is in their court,” he says in the trailer released on Thursday (5 January).
Follow live updates about Harry’s memoir here.