Mr Kay also referenced Diana’s wishes for Harry and William to be treated equally, despite the Sussex royal being viewed as “spare to the heir”.
He said in the documentary: “Prince Charles shared Diana’s concerns about making sure the boys shared things in their early years.
“But, certainly, more senior members of the royal family in private were saying: ‘This is nonsense. William is the future. All our efforts must go into educating William’.
“It was William the Queen would invite for lessons in kingship when he was at Eton and she was at Windsor Castle.”