The Queen has "had several conversations" with William about helicopter safety. William is a trained helicopter pilot and has been known to sometimes fly his family from Kensington Palace to their home in Norfolk.
"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 has told William she is worried that, however good he is as a pilot, bad weather and accidents can strike at any time."
The Cambridges recently shared their annual Christmas card image.
Released on their Twitter account, the image shows the couple and their three children, Prince George, 8, Princess Charlotte, 6, and Prince Louis, 3, while on a private trip to Jordan.
They tweeted: "Delighted to share a new image of the family, which features on this year's Christmas card."
The family, snapped sitting in front of a red rock, shows George and Charlotte perched on individual chairs and their brother, Louis, sitting on a rug, crossed legged.
Behind the trio, William sits on a small gold seat next to Kate, who has positioned her hand on her husband's knee.
A grown-up looking George wears a camouflage-print polo shirt while Charlotte is in a gingham blue dress and little Louis wears a grey polo shirt.
