Back in 2014, Prince William and Princess Kate travelled Down Under, taking baby Prince George with them.

Prince William and Princess Kate broke an unusual yet extremely important royal rule when they took Prince George with them on a tour of Australia and New Zealand.

The young family made the trip in the spring of 2014, less than a year after George’s birth in July 2013, but although it seemed like a sweet opportunity for some family bonding, they would have needed to seek the permission of Queen Elizabeth first.

This is because heirs to the throne are generally not permitted to board the same plane in case of an accident, which could destabilise the monarchy.

With two royal heirs travelling together, the late Queen would have had to give her blessing for the trip a decade ago, and next year the rule will affect George even more as he turns 12.

From that date he will no longer be able to travel with his father William nor his younger sister Princess Charlotte, the next in line to the throne after him, again in case of a plane crash.

Princess Kate and Prince William travelled to Australia in 2014 (Image: Getty)

William too had to stop travelling with then-Prince Charles, Princess Diana and Prince Harry when he turned 12 in 1994 for the same reason.

The King’s former pilot Graham Laurie previously told OK! magazine: “We flew all four: the Prince, the Princess, Prince William and Prince Harry, up until Prince William was 12 years old.

“After that, he 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. When William became 12, he would fly normally in a 125 from Northolt and we would fly the 146 out with the other three on.”

Kate and William took George with them despite the royal rule (Image: Getty)

There is another unexpected rule in place for the Royal Family when they travel, and that is that they must carry blood with them matched to the Firm’s blood type.

Dickie Arbiter, former press secretary to the late Queen, explained on Hello’s A Right Royal Podcast: “[You’ve] got to make sure that you’re covering every eventuality.

“The aircraft would carry blood in case there was a blood transfusion. There’s never any guarantee you are going to get the right type of blood at your destination.”

Soon George will not be able to travel with William and Charlotte (Image: Getty)

In 2016, it was reported by The Telegraph’s Gordon Rayner that the late Queen would bring a Royal Navy doctor with her on royal tours in case of a medical emergency.

The doctor’s job was also to research local hospitals and medical facilities.

He said: “In countries where a reliable blood supply is questionable, the Queen and the Prince of Wales both travel with their own personal packs of blood following in their convoy wherever they go.

“Their doctor is never more than a few paces away, carrying a bulky medical bag containing a mobile defibrillator and all manner of emergency medicine.”