A man armed with a crossbow who was arrested after attempting to break into Windsor Castle where Queen Elizabeth was celebrating Christmas is being held under the Mental Health Act, London's police said on Sunday.

The police said the 19-year-old from Southampton in southern England was stopped within moments of him entering the grounds of the castle on Christmas morning and did not break into any buildings.

"The man was taken into custody and has undergone a mental health assessment - he has since been sectioned under the Mental Health Act and remains in the care of medical professionals," the police said. "Following a search of the man, a crossbow was recovered."

Queen Elizabeth, who has spent much of the COVID-19 pandemic at Windsor Castle, is spending Christmas there with her son, Prince Charles, his wife, Camilla, and other close family.

The arrested teenager used a rope ladder to scale a metal fence, according to the Mail on Sunday.

Security breaches at royal residences are rare. The most serious one in the queen's reign happened in 1982, when an intruder climbed a wall to enter Buckingham Palace, her London home, and wandered into a room where she was in bed.

The queen made no public appearance on Saturday. In a pre-recorded Christmas message to the nation, she spoke of the loss of her husband, Prince Philip, who died in April aged 99. The couple were married for 73 years.