Princess Diana's secret voice coach claims 'Lilibet' is a tribute to Prince Harry's late grandfather - not the Queen.
The nickname was a pet endearment used by Prince Philip for the monarch.
Harry will have chosen the name after his grandfather's death because he adored him, Stewart Pearce says.
The Duke of Edinburgh was a pillar for the young prince after his mother's tragic death, he told the Mail Online.
He continued: "There is no doubt that the influence Prince Philip had over his grandson will live on in the heart of Harry, in the sense of the complete allegiance Prince Philip's life created in alliance with the Queen, after the sudden death of her father the King, and her own accession to the throne."
Harry and wife Meghan Markle welcomed baby Lilibet Diana Mountbatten-Windsor into the world on June 4.
She joins older brother Archie in the family's new life across the Atlantic.
Lilibet was born barely two months after Philip's death at the age of 99. He had a 73-year marriage with the Queen.
Mr Pearce said Harry, who has been open in his devotion to Meghan, had a lot in common with his grandfather.
"After all Philip gave up a highly promising Naval career to be by the side of the woman he loved," he said.
Mr Pearce, a vocal expert whose 2020 book, Diana: The Voice of Change, documented his experiences with the princess from 1995 to 1997, said he believed she would have been "thrilled" by her granddaughter's name.
He said: "I write in my book that Diana gave permission for the formal behaviour that stultifies love to be reinvented, for emotional aloofness to be made transparent, for starched stuffiness to be given a human face, for feeling expression to be given its rightful place, and for dismissive criticism to be turned into discerning care."
The couple's choice of the Duke's pet name for the Queen was viewed as a tribute to their daughter's great-grandmother.
However it fuelled speculation over whether she gave her blessing for the special nickname to be used.
Mr Pearce claimed he had heard from his royal sources that the Queen was "overjoyed" at Lilibet's name.
The Queen has endured a fractious year after Meghan and Harry announced their split from royal life.
Their departure is widely believed to have deepened Harry's rift with brother Prince William.
Harry and Meghan's explosive Oprah Winfrey interview then only fanned the flames.
This week brought yet more discomfort for The Firm, as Prince Charles' financial records were laid bare, in the release of his annual report by Clarence House showing that he gave the Sussexes and Cambridges a share of £4.45million..
The financial records cast doubt on Harry's assertion in the couple's Oprah interview that his father had 'cut him off', leaving him forced to sign lucrative commercial deals to support his family.
Harry had said his Netflix deal thought to be worth millions was never part of the plan, telling Oprah: "That was suggested by someone else by the point of where my family literally cut me off financially and I had to afford security for us."
The Clarence House report said the money was given to "fund the activities" of Harry, Meghan, William and Kate.
A spokesperson for the Sussexes said there is no contradiction between Prince Harry's claim and the money his father gave the couple.
They explained Harry was referring to the period running from April to July - which is the beginning of the financial year in the UK.