William and Kate waited until George's 7th birthday to tell him he'll be King one day

Prince George is third in line to the throne, but it turns out that the little royal didn't find out about his future duties until last summer when he turned seven years old.

In an updated version of his book Battle of Brothers, which has been released today, royal author Robert Lacey writes that his parents, Prince William and Kate Middleton, wanted to talk to their eldest son about his future role at a 'controlled moment of their choice'.

He said: '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.

'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".'

Discussing the famous 'Four Monarchs' photo taken in December 2019, featuring the Queen, Prince Charles, Prince William and Prince George, Lacey continued: 'The prince had been placed on a rep-carpeted step in order to bring him up closer to the level of his great-grandmother.

'What a fascinating and historic image to remind us of the essence of the royal system! The current monarch with three future monarchs. All the living heirs - and not a suggestion of a "spare".

'According to insiders, this formal photograph, taken in the Buckingham Palace Throne Room a week before Christmas 2019, was the idea of Prince Charles, anxious to promote his cause of the "slimmed-down monarchy".'

Royal expert Katie Nicholl has spoken about George's upbringing in the past, telling OK! magazine that the young royal is 'singled out' to spend time with the Queen and Prince Charles, and that he 'understands that he's different from them'.

However, she also claims that the Duke and Duchess of Cambridge ensure that they treat the three little ones as equals, and that George doesn't receive special treatment compared to his younger siblings.