Queen Elizabeth II has urged Prince William, her grandson and second heir to the British throne, to stop flying helicopters himself with his family on board because she fears that disaster could strike.

Prince William, who served as a full-time pilot with the East Anglian Air Ambulance from July 2015 for two years, regularly flies between his London residence Kensington Palace and Norfolk country home Anmer Hall with his wife Kate Middleton and their three children Prince George, Princess Charlotte, and Prince Louis.

The Duke of Cambridge is second in line of succession to the British throne behind his father Prince Charles, while his eldest son George is expected to take over the throne after him. The traditional protocol barred senior royals from flying together as a precautionary measure in case there is a tragedy, but the unofficial rules were relaxed on the request of William after the birth of his first child in 2013, making it possible for him to fly with his heir on board.

However, the Queen continues to hold concerns regarding the matter and is "terrified" that a fearful disaster could strike and damage the line of succession. She has had "several conversations" with William where she requested him to "stop flying himself, particularly in bad weather" conditions, reports The Sun.

A source close to the monarch told the outlet, "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."

The insider added that the matter keeps the 95-year-old awake at night and she is "understandably very worried" about it. They explained, "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 source noted that the monarch is "delighted" with how William, as well as his wife Kate Middleton, have risen to the challenge in recent years, like the exit of Prince Harry and Meghan Markle as working royals. She "knows the monarchy is safe in their hands" and thinks "the future is bright with them at the helm after Charles."

However, she can't bear thinking about what would happen "if something happened to him and the family."

Prince William is serving as a search and rescue helicopter pilot, based at RAF Valley on Anglesey (Reuters)
Prince William served as a search and rescue helicopter pilot, based at RAF Valley on Anglesey (Reuters)