Prince Harry always revelled in the freedom he enjoyed as a child - something he knew brother William would never have, a royal author claims.
Robert Jobson said the "mischievous" little royal was "sometimes totally out of control" and told his older sibling, and second in line to the throne, he could "do what I want".
The now-Duke of Sussex, 37, often felt overshadowed by the attention William received, including being treated with deference by royal staff at a young age.
He would also be invited to visit the Queen Mother alone and without his younger sibling.
The author and journalist said while Harry's never directly expressed resentment in public it did all cause some tension between the two of them.
But this came out in shows of cheeky rebellion that their mum, Princess Diana, found "hilarious".
On one occasion, the three of them, alongside royal protection officer Ken Wharfe, were being driven to Prince Charles' home at Highgrove, in Gloucestershire.
Harry and William, just six and eight, were arguing on the back seat when the younger brother said: "You'll be King. I won't - so I can do what I want!"
Jobson, writing in his new book William At 40: The Making Of A Modern Monarch, extracts of which are being published in the Daily Mail, said Diana and Wharfe were "a little shocked", the protection officer later told him.
The author said: "Certainly, Harry seemed to revel in the freedom that he knew William would never have. He was far more mischievous — and sometimes totally out of control."
And while William was "also capable of wild behaviour" he could become "irritated" by how far Harry would go, often acting as a "tattle-tale" on his brother.
Diana, on the other hand, was keen to embrace such rebellion.
On one occasion, when Harry got in trouble at school for pulling on the music teacher's trousers while he played the piano, she "burst into a fit of giggles" when she had to go in to discuss his behaviour.
"The truth is that the princess actively encouraged Harry’s mischievous nature, which was in some ways akin to her own," said Jobson.
Once they arrived at Highgrove after Harry's outburst at William, Diana encouraged them to have a go-kart race and tear up Charles' immaculate grounds.
Their dad was away at the time.
Jobson said Diana "believed in letting her boys run wild and free" and they would "roam around" Highgrove's grounds "like a couple of urchins".
Charles and Diana would often leave the two boys in the care of nannies and protection officers, who were in charge of discipline, with Harry once upsetting one of the former by telling her to lose weight.
Wharfe recalled how they both would regularly team up to punch him in the head and groin during playful wrestling matches.
However, he wasn't as lenient as others in his role.
He remembered an outgoing officer telling his replacement "good luck" and that if William and Harry had been brought up on a council estate "they'd have been taken into care by now".
