A man armed with an "offensive weapon" has been arrested in the grounds of Windsor Castle on Christmas Day.
Thames Valley police said the man, aged 19 and from Southampton, was found within "moments" of entering the grounds at 8.30am (UK time).
He has been arrested on suspicion of breach or trespass of a protected site and possession of an offensive weapon.
The force said he did not enter any buildings and members of the Royal Family had been informed.
Thames Valley police Superintendent Rebecca Mears, said: "An investigation is ongoing following this incident and we are working with colleagues from the Metropolitan police.
"We do not believe there is a wider danger to the public."
The Queen is at Windsor Castle, rather than spending Christmas as usual on her Sandringham estate.
The 95-year-old monarch, who has spent much of the Covid-19 pandemic at Windsor Castle, was celebrating Christmas there with her son, Prince Charles, his wife, Camilla, and other close family.
Charles, Camilla and other royal family members were pictured later in the morning arriving for a Christmas church service at St George's Chapel within the Windsor Castle complex.
There was no suggestion that any of the royal family's plans had been disrupted by the incident.
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.
In 2016 a man with a previous murder conviction pleaded guilty to trespass after scaling a perimeter wall at Buckingham Palace and asking if the monarch was at home.
The royal family normally spend Christmas at the queen's Sandringham estate in eastern England, but that tradition has been disrupted by the Covid-19 pandemic. She spent Christmas of 2020 at Windsor, too.
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.
-BBC / Reuters