British police arrested a man armed with a weapon on the grounds of Windsor Castle on Saturday.
Thames Valley police said the security breach took place at 8.30am on Christmas Day.
The 19-year-old from Southampton is in custody.
“The man has been arrested on suspicion of breach or trespass of a protected site and possession of an offensive weapon. He remains in custody at this time,’’ Thames Valley Police Superintendent Rebecca Mears said.
“We can confirm security processes were triggered within moments of the man entering the grounds and he did not enter any buildings."
Members of the royal family have been informed.
Queen Elizabeth II is spending the Christmas holidays at Windsor, having decided not to stay at Sandringham, the royal estate in eastern England where she traditionally spends the festive season with family.
Queen Elizabeth delivered a particularly poignant Christmas message yesterday, devoting her annual address to the legacy of Prince Philip, who died in April.
From the grounds of Windsor castle where Queen Elizabeth is spending her first festive season without Philip, the 95-year-old monarch delivered a Christmas message tinged with sadness.