The Queen has urged her grandson Prince William to stop flying helicopters with Kate Middleton and their children over safety fears, it has been reported.
Her Majesty is said to be "terrified" an accident could happen, threatening William's succession to the throne.
An unwritten rule already prevents senior royals from flying together just in case disaster strikes.
The protocol was relaxed as Prince William's family grew bigger and they began to split their time between London and Norfolk.
But a source close to the Queen said she is "terrified" something could go wrong and has now spoken to William about her worries, reports the Sun.
Prince Charles, 73, is next in line to the throne after the Queen, before William, 39, and his eldest son Prince George, eight.
William has previously flown his family 115 miles from their London home at Kensington Palace to Anmer Hall in Norfolk.
The Queen's concerns come after two near-misses in one royal helicopter have led to a major probe being launched.
A source close to the Queen claimed: “Her Majesty has told close friends and courtiers that she would like William to stop flying himself, particularly in bad weather, as helicopters are not the safest form of transport.
“It keeps the Queen awake at night and she is understandably very worried.
“She knows William is a capable pilot but does not think it is worth the risk for all five of them to carry on flying together and can’t imagine what would happen. It would spark a constitutional crisis."
The Queen reportedly thinks the future is "bright" with William and Kate having risen to the challenge of senior royals in recent years, and something bad happening to them "doesn't bear thinking about".
Back in May, Prince William was pictured walking to his helicopter with his wife and three children.
According to reports, they often fly together in a helicopter with a crew and pilot and sometimes William gets behind the controls too.
Her Majesty's concerns come after a couple of recent helicopter incidents involving royals.
The Queen’s Sikorsky S-76 helicopter had a fault in September and in the same month a planned flight was cancelled because of bad weather.
Sophie Wessex had a near miss in a chopper in 2017 and then two years later the Duchess of Cornwall saw two serious incidents during one flight in the Queen's helicopter.
Firstly it had to swiftly veer off course after coming within 200ft of an aircraft carrying parachutists. Later in the journey it had to try to avoid a glider near the Duchess's home at Highgrove.
A review into safety around royal air travel has now been launched by the Civil Aviation Authority.
Technically Prince William should never travel on a flight with his father Charles or his children.
But then in 2014, after Prince George was born, William and Kate asked for the Queen's permission to all fly together when they went on their tour of Australia.
It's thought the protocol has been relaxed even more since Charlotte and Louis were born.
Kate and William are rumoured to be searching for a house near Windsor so they can be nearer the Queen and schools for their children, and remove the need for flying in a helicopter so much.
William graduated from his RAF training course in 2010, qualifying as a pilot. He made brave rescues in Sea King helicopters and then qualified as a captain in 2012. He was a pilot for East Anglian Air Ambulance from 2015-2017.