A serious rift developed between William and Harry when the younger brother was pictured in a Nazi costume, according to a new book.
The siblings went to Maud's Cotswold Costumes in January 2005 and chose an animal outfit for William and, for Harry, a khaki-coloured uniform, in which the young duke was photographed while displaying an armband with a swastika.
The response to the incident prompted Harry to reconsider his older brother's involvement and the differing treatments of the pair, noted royal biographer Robert Lacey claims in Battle of Brothers, which is being serialised in the Daily Mail.
The book says Harry began to feel alienated from his family following the costume incident, with a former aide saying of the brothers: "For the first time, their relationship suffered and they barely spoke. Harry resented the fact that William got away so lightly."
Mr Lacey also claims the boys' nanny Barbara Barnes acted as a surrogate mother due to Diana's health concerns and humanitarian commitments, with Ms Barnes teaching them to walk, talk and read.
She was later dismissed without being allowed to say farewell to the young princes.
Mr Lacey wrote: "Following the death of Diana, people remarked on how well the two young princes reacted to the unjust and unexpected removal of a mother figure from their lives - surprised, bewildered and distraught though they were.
"In fact, 10 years earlier, they'd had a little practice."
The book also alleges that the Queen felt the Duke and Duchess of Sussex were "erratic and impulsive" in making their departure;
It also claims the royal family were "hopping mad" over Harry and Meghan Markle's trademarking of Sussex Royal products and services, which was viewed as a "commercialising of the crown".
The Queen, Prince Charles and William were not consulted over the multiple legal actions taken by the couple against the media, according to the book.
It also alleges that a "powerful constituency" within the family felt Meghan's interview with ITV's Tom Bradby during last year's tour of Africa, in which she bemoaned her life, showed a "bizarre tone deafness" and was "miserably self-indulgent".