A man has been sectioned under the Mental Health Act after being found in the grounds of Windsor Castle with a crossbow, police have said.
The Metropolitan Police said a 19-year-old man from Southampton was arrested at about 08:30 GMT on Christmas Day.
It said the man was stopped “within moments” of entering the grounds and he did not enter any buildings.
He was searched and a crossbow was found, police said. The man is now “in the care of medical professionals”.
He had initially been arrested on suspicion of breach or trespass of a protected site, and possession of an offensive weapon.
The Queen has been staying at Windsor Castle, rather than spending Christmas as usual on her Sandringham estate in Norfolk.
Police had previously said the Royal Family had been informed of the incident.
Inquiries into the “full circumstances” of what happened would be continued by the Metropolitan Police Specialist Operations, they said.