The Duke and Duchess of Cambridge have reportedly been spotted boarding a plane at Heathrow airport for a holiday with their children, however, a traditional royal rule may spell an end to the royals travelling on the same plane or in the same vehicle when Prince George turns 12.

According to a centuries-old law, senior royals are prohibited from travelling together in an effort to protect the future of the monarchy. Heirs to the throne are unable to travel on the same mode of transport in case of a tragic accident. Prince Charles is unable to travel with his mother, the Queen and Prince Willam is prohibited from travelling with his father, Charles.

In some cases, the Queen can grant permission to the family to fly together, and travelling with children has lately been granted. Prince William and Kate brought George to New Zealand in 2014 but had to receive special permission from the Queen to do so.

A Royal Spokesperson explainedto BBC Newsround the process to bring George to New Zealand in 2014.

“They (William and Kate) had to ask the Queen for permission, but she said yes. While there is no official rule on this, and royal heirs have travelled together in the past, it is something that the Queen has the final say on,” the spokesperson said.

Prince George is third in line to the throne, behind Prince Charles and his dad Prince William. Once George turns 12 in four years he will be expected to travel separately from his father, Prince William.