Marlene Koenig told the Sun: 'The sovereign has legal custody of the minor grandchildren.
'Legislation passed during the reign of George I. It was known as The Grand Opinion for the Prerogative Concerning the Royal Family and it was about the King’s control over the education, the raising and the marriage of his grandchildren.
'He did it because he had a very poor relationship with his son, the future King George II, so they had this law passed that meant the King was the guardian of his grandchildren.'
The bizarre law dates back over 300 years to 1717 when the monarch's 'right of supervision extended to his grandchildren and this right of right belongs to His Majesty, King of the Realm, even during their father’s lifetime,' the Sun claims.
This also means that when Queen Elizabeth passes away, the custody of royal minors would pass onto Prince Charles.
Marlene advised that the law still exists today, and has recently affected the way the royals parent their children.
Koenig says, 'When Harry was an infant, Charles asked the Queen if he and Diana could travel with both kids to Scotland (on a plane). The Queen said yes.
'Later, as Harry got older, he would fly with parents, and William would travel separately.
'Technically, they needed permission for travel. The Queen has the last word on parenting decisions like that.'
Michael Nash, a constitutional expert, wrote in The Times in 1993 that the 'Queen has the last word in the custody upbringing, education and even the right of abode of the princes, even during the lifetime of their father, Prince Charles.
'As for their mother, the Princess of Wales, her say is a matter of discretion and negotiation.'
While the law is technically still in existence, Marlene tells the Sun it typically is a 'formality and nothing more.'
She adds, 'The grand opinion has never been changed, but the Queen has a far better relationship with her kids than George I had with his son, the future George II.'