Senior members of the Royal Family should avoid travelling together in case there is an accident.
Prince William, as the heir to the throne, should not fly with his father, Prince Charles, or any of his children, according to the unwritten rule.
But William and Kate have been granted permission by Her Majesty to travel with their children.
In 2014, the royal couple were allowed to board the same plane as Prince George, who was a baby, when they took off for their tour of Australia.
The royal press office confirmed the arrangements made for the unofficial rule to BBC’s Newsround.
The spokesperson said: “They had to ask the Queen for permission, but she said yes.
“While there is no official rule on this, and royal heirs have traveled together in the past, it is something that the Queen has the final say on.”
According to Elle, the Queen may have felt more confident in the safety of aircrafts than she did decades ago, hence why she allowed the Cambridges to bypass the protocol.
The senior royal aide said: “The current plan is for the Queen’s birthday parade to go ahead in London as normal, with the acceptance that it may need to be adapted or scaled back depending on what guidelines are in force at the time. But the aspiration and the ultimate desire is to make it happen.”
However, it is currently unknown whether Government measures to fight the pandemic will allow parades to happen in 6 months’ time.
Trooping the Colour, the Queen’s official birthday, will be celebrated on June 12 this year.
Royal aides told the Sunday Times the parade will serve as both an official and “family occasion”.
The event has led to speculation regarding Meghan Markle and Prince Harry’s potential return to the UK.