The rule-breaking can be seen every time the family travel somewhere; they usually arrive together. A royal family principle prevents heirs to the throne travelling together in case of an accident or an emergency. The rule dates back to the early days of air travel when planes weren’t as safe and accidents often happened.
Kate married Prince William, second in line to the throne, in 2011.
Their eldest son, George, is third in line to the throne.
The Royal couple’s two other children, Charlotte and Louis, are fourth and fifth in line to take the crown, respectively.
The family have been seen to break the royal protocol on an number of occasions.
The first time the royal parents broke the rules was in 2014 when Prince George was set to head off to New Zealand and Australia for a quick tour with his parents.
The rules dictated George would travel separately from his father, but they all took the same plane.
Since then, the family has always travelled together.
Last month the family were seen flying on a commercial airline to visit Scotland.
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The family were thought to be visiting the Queen at Balmoral.
Another stunned onlooker told The Sun: “Earlier in the flight, I’d had the feeling that someone had come on the plane that I’d recognised – but I thought it was just a footballer.
“We landed, and people then saw two black Range Rovers on the tarmac. Suddenly, Will, Kate, and the kids all got up, and left.
“They weren’t harassed by anyone at all as they got off the plane. They just seemed like a family travelling together to be honest.
“No one saw them get on the plane either. It was amazing how they did it – they must have slipped on quietly, after people were already seated.”
As well as flying the Cambridge’s are often seen arriving places together by car.
Last week Prince William drove George to his first day back at school.
They were accompanied by Kate and Charlotte, who started at the school the same day.
Both William and Kate have been praised for their “down-to-earth” parenting.
A royal insider said: “The Cambridge kids are really lively. Clearly the kids are having fun.”
They called Prince William “a very modern dad”, and added Kate was “a great mother”.