Prince William, 36, and Prince Charles, 69, still have to speak to their grandmother and mother respectively if they want to travel with one another.
The Queen, 92, will need to give formal permission to the father and son should they wish to fly in a plane together.
This royal rule is to prevent the heirs to the British throne from travelling together at the same time.
This is so if there is an emergency the lineage of the royal line will not be affected and the country will still have a ruler.
However, Prince William has regularly broken this rule when travelling with his own son.
Prince George, 5, is the third in line to the throne but has previously flown in a plane with William, who is second in line to the throne.
William and Kate Middleton, 36, travelled to Australia and New Zealand in 2014 with Prince George.
They also visited Canada in 2016 on a royal tour, which both Prince George and Princess Charlotte, 3, joined them on.
The same rule applies to Princess Charlotte as she is also in line to the throne.
Kate and William are believed to have enjoyed their first holiday as a family of five in Mustique last month, and Prince Louis, 3 months, joined them – proving the royal couple will be ignoring the royal rule for all their children.
It is thought that Queen Elizabeth permitted Prince William to fly with his children and wife.
“They had to ask the Queen for permission but she said yes,” said the Royal press office.
Prince Charles and Princess Diana also didn’t abide by the protocol dictated by the Royal Family and frequently travelled with Prince William and Prince Harry inflight.
Meghan Markle, 37, and Prince Harry, 33, are also unlikely to be separated from their future offspring when travelling abroad.
The newest royal couple has broken protocol before when travelling between countries.
In 2016 when the couple began dating, Prince Harry was due to travel back to London after a Caribbean tour.
Instead, he chose to travel to Canada to see Meghan, changing the flight route.
While he would have paid for the plane ticket himself, this is frowned upon as the Royal Family dictate: “Combining private and public engagements in the same trip is actively discouraged”.
This was following an uproar in 2004 after Prince Andrew mixed work trip travel with his personal trips.
For personal trips, the royal family are often not permitted to fly using royal expenses.
