Prince William and Kate Middleton often break an unwritten royal protocol when travelling by plane together.

Though not official, royal protocol discourages two or more heirs from taking the same flight to minimise risk to the royal lineage.

The Duke and the Duchess of Cambridge frequently fly with their three young children, Prince George, Princess Charlotte, and Prince Louis, who are third, fourth and fifth in line to the throne respectively.

Prince William is currently second in line to the throne after his father, Prince Charles, meaning he should technically be flying in separate planes to his children.

Prince William, Kate Middleton and family
Prince William and Kate Middleton frequently break royal rule while flying

Ordinarily, the rule would also prevent any of the couple’s three children from travelling together, however, the rule has been relaxed in recent years as safety in the aviation world has improved.

In 2013, Prince William asked Her Majesty for special permission to break the rule, and Prince George subsequently accompanied the family on their royal visit to New Zealand in 2014.

With the arrival of Princess Charlotte in 2015, the family continued to disregard the guidance, travelling together to Germany, Poland and Canada.

Duke and Duchess of Cambridge
The Cambridges often travel together

A royal spokesperson told the BBC: “While there is no official rule on this, and royal heirs have travelled together in the past, it is something the Queen has the final say on.”

Since the birth of Prince Louis, however, the family has done no official travel together. Princess Charlotte and Prince George are now in school.

The family has continued to go on holiday together to Mustique in the Caribbean.