According to the book The New Royals, author and Vanity Fair royal correspondent Katie Nicholl said the cheeky prince, nine, had used the line on a classmate who’d gone too far.

She said Princess Charlotte, seven, and Prince Louis, four, were being raised with an understanding of how the monarchy worked and also “a sense of duty”.

She said George had been told he would one day be king, but the Prince and Princess of Wales were not weighing the children down with too many responsibilities too soon.

However, George had already weaponised the potential title, using it against one of his schoolmates as a “cheeky comeback”.

“George understands he [William] will one day be king and as a little boy sparred with friends at school, outdoing his peers with the killer line: ‘My dad will be king so you better watch out’.”

George was reportedly told of his future role as a monarch in a “controlled moment” of William and Kate’s choice, which was believed to have been “sometime around his seventh birthday”.

“William’s aim as a father, the prince stressed, was to give his son ‘a normal family upbringing’, enabling the monarchy ‘to stay relevant and keep up with modern times’.”

The young prince was given a role at Queen Elizabeth II’s state funeral, walking behind Her Majesty’s coffin alongside his sister Princess Charlotte.