• Prince William is already preparing Prince George to one day assume the throne, according to royal historian Robert Lacey
  • The duke told his eldest son about his "life of future royal service and duty" last year, the author's book says
  • Prince William reportedly wanted to give his son a "normal family upbringing"

Prince George already knows his future role in the monarchy, a new book has claimed.

Royal historian Robert Lacey published new chapters for his book "Battle of Brothers." In an excerpt published by the Daily Mail, the author claimed that Prince William and Kate Middleton told their son that he will be king one day around the time he turned 7 years old in July last year.

"William has not revealed to the world how and when he broke the big news to his son. Maybe one day George will tell us the story himself. But sometime around the boy's seventh birthday in the summer of 2020, it is thought that his parents went into more detail about what the little prince's life of future royal 'service and duty' would particularly involve," Lacey wrote in the excerpt.

"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,'" he continued.

The way Prince William handled sharing the news with his son was in part due to the Duke of Cambridge's "unhappiness at the haphazard fashion in which the whole business of his royal destiny had buzzed around his head from the start," the biographer wrote.

If the author is to be believed, Prince George was not yet aware of his role in the monarchy when he joined Queen Elizabeth II, Prince Charles and Prince William in posing for the "Four Monarchs" portrait in December 2019, Daily Mail noted. Clarence House shared the same photo on Instagram in January 2020.

"To mark the start of a new decade, a new portrait of Her Majesty The Queen, The Prince of Wales, The Duke of Cambridge and Prince George has been released," read the caption of the image, which was taken by Ranald Mackechnie in the throne room of the Buckingham Palace.

In a 2019 interview, Prince William said he had not yet spoken with Prince George about his future as a king. The royal father-of-three said he has not opened up about the matter to his eldest son because, at the time, he felt that he was still too young and that it wasn't the right time.

“There’ll be a time and place to bring George up and understand how he fits in, in the world. But right now it’s just a case of keeping a secure, stable environment around him and showing as much love as I can as a father,” Prince William told the BBC.

The Duke of Cambridge added that they were a "normal family." He also hoped that his children would see how much he loves them.

“I love my children in the same way any father does, and I hope George loves me the same way any son does to his father,” Prince William said.

Prince William and Middleton are also parents to Princess Charlotte, 6, and Prince Louis, 3.

Prince George 2019
Prince George of Cambridge attends the Christmas Day Church service at Church of St Mary Magdalene on the Sandringham estate on Dec. 25, 2019, in King's Lynn, United Kingdom. Pool/Samir Hussein/WireImage