As Prince George, the eldest son of Prince William and Kate Middleton, turns 12 next year, he will have to follow a strict royal protocol that bans him from flying on the same plane as his father. This rule, deeply rooted in royal tradition, aims to secure the future of the British monarchy.

On Monday, Prince George celebrated his 11th birthday, marking the last year he can enjoy traveling with his father, the Prince of Wales. Veteran journalist and royal biographer Angela Levin discussed this upcoming change on GB News, explaining the significance of the rule.

"When he gets to 12, he can't fly in the same plane as his father," Levin told hosts Eamonn Holmes and Ellie Costello. "You can't have an heir to the throne in the same plane as another heir," she explained, noting the sadness of this separation. "I think that's very sad, so he's got to enjoy it very much."

The rule that prevents two direct heirs to the throne from traveling together was established to ensure the stability of the monarchy in the event of a plane crash. This precautionary measure has historical precedence, given the tragic deaths of three royals in plane crashes: Princess Cecile, Prince Philip's sister, in 1937; Prince George, Duke of Kent, in 1942; and Prince William of Gloucester, who died in an air show accident in 1972.

The Mirror U.K. reported that the rule can be broken with the monarch's permission, adding a layer of flexibility to the otherwise rigid protocol. For instance, King Charles' former pilot, Graham Laurie, shared insights into how this rule has been applied in the past. "Interestingly, we flew all four: then-Prince Charles, Princess Diana, Prince William, and Prince Harry, up until when Prince William was 12 years old," Laurie told Hello! magazine's "A Right Royal Podcast." After that age, Prince William traveled separately, necessitating written permission from the Queen to fly together.

Laurie elaborated on the logistics of royal travel, explaining that Prince William would typically fly in a 125 aircraft from Northolt, while his parents and younger brother flew in another plane. This arrangement underscored the importance of ensuring that multiple heirs are not put at risk simultaneously.

In the present day, Prince William and Princess Kate continue to honor this tradition. To celebrate Prince George's 11th birthday, they shared a new black-and-white portrait of the young prince on Instagram. The photo, taken by the Princess of Wales earlier this month, shows Prince George smiling in a dark suit and white button-down shirt.

Levin revealed more details about Prince George's birthday celebrations, noting that he will have two parties this year. One event will be a family affair in London, while the other will be a larger gathering with all of his school friends. These celebrations highlight the balance the royal family strikes between upholding traditions and ensuring a joyful upbringing for the young prince.