Prince William and Kate Middleton's eldest son, Prince George, will have to follow a strict royal rule when he turns 12 next year.

On Monday, Prince George -- who is second in line to the British throne after his dad -- turned 11, which means he has just one more year left to enjoy flying with Prince William.

According to veteran journalist and royal biographer Angela Levin, Prince George will be banned from flying in the same plane as the Prince of Wales after he celebrates his 12th birthday.

"When he gets to 12, he can't fly in the same plane as his father," Levin told GB News hosts Eamonn Holmes and Ellie Costello.

Royal family
Britain's Prince George of Wales, Britain's Prince William, Prince of Wales, Britain's Prince Louis of Wales, Britain's Catherine, Princess of Wales, Britain's Princess Charlotte of Wales, Britain's King Charles III and Britain's Queen Camilla wave on the balcony of Buckingham Palace after attending the King's Birthday Parade "Trooping the Colour" in London on June 15, 2024. HENRY NICHOLLS/AFP via Getty Images

"You can't have an heir to the throne in the same plane as [another heir]," the royal expert explained. "I think that's very sad, so he's got to enjoy it very much."

The Mirror U.K. reported that the rule that two direct heirs to the throne cannot travel in the same plane can be broken if the monarch gives permission.

The rule was established to secure the future of the monarchy, which would become "unstable" in the event of a crash, according to the outlet.

England v Wales - Guinness Six Nations
LONDON, ENGLAND - FEBRUARY 26: Catherine, Duchess of Cambridge speaks to their son Prince George of Cambridge prior to the Guinness Six Nations Rugby match between England and Wales at Twickenham Stadium on February 26, 2022 in London, England. (Photo by David Rogers/Getty Images)

Three royals have been killed in plane crashes, including Prince Philip's sister Princess Cecile in 1937 and Queen Elizabeth II's uncle Prince George, Duke of Kent in 1942.

Prince William of Gloucester, a cousin of the late monarch, also died while competing in an air show in 1972.

Last year, King Charles' former pilot, Graham Laurie, revealed that Prince William stopped flying on the same plane as his father after turning 12 in 1994.

"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."

TOPSHOT-BRITAIN-ROYALS-CORONATION-CONCERT
TOPSHOT - Britain's Prince George of Wales (L) and Britain's Prince William, Prince of Wales react as Britain's Prime Minister Rishi Sunak (2R) and his wife Akshata Murty attend the Coronation Concert at Windsor Castle in Windsor, west of London on May 7, 2023. For the first time ever, the East Terrace of Windsor Castle will host a spectacular live concert that will also be seen in over 100 countries around the world. The event will be attended by 20,000 members of the public from across the UK. (Photo by Stefan Rousseau / POOL / AFP) (Photo by STEFAN ROUSSEAU/POOL/AFP via Getty Images)

"After that, [William] had to have a separate aircraft, and we could only fly all four together when they were young with the written permission of Her Majesty," he added.

According to the former royal pilot, Prince William "would fly normally in a 125 from Northolt," while his mother, father, and younger brother would fly on another plane.

Meanwhile, Prince William and Princess Kate marked Prince George's 11th birthday Monday by sharing a new photo of him on Instagram.

The black-and-white portrait, which was taken by the Princess of Wales earlier this month, showed the young prince smiling as he posed in a dark suit and white button-down shirt.

Levin told GB News that Prince George will have two parties this year.

One bash was to be held in London with family and close friends. The other is a "big one" that will be held later so Prince George can celebrate with "all of his school friends."

Tags
Prince George, Prince William