A man armed with a weapon was arrested in the grounds of Windsor Castle as Queen Elizabeth II celebrated Christmas there.
Thames Valley police said the security breach took place at 8.30am on Saturday.
A 19-year-old man from Southampton has been arrested on suspicion of breach or trespass of a protected site and possession of an offensive weapon, detectives say.
Newspaper reports claim the arrest came after security controllers monitoring CCTV spotted a man carrying what appeared to be a crossbow.
“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 Supt 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 usually spends the festive season with family at Sandringham, the royal estate in eastern England.
She is staying at Windsor Castle this year because of the pandemic.
Mohammed bin Zayed Majlis
Farage on Muslim Brotherhood
Nigel Farage told Reform's annual conference that the party will proscribe the Muslim Brotherhood if he becomes Prime Minister.
"We will stop dangerous organisations with links to terrorism operating in our country," he said. "Quite why we've been so gutless about this – both Labour and Conservative – I don't know.
“All across the Middle East, countries have banned and proscribed the Muslim Brotherhood as a dangerous organisation. We will do the very same.”
It is 10 years since a ground-breaking report into the Muslim Brotherhood by Sir John Jenkins.
Among the former diplomat's findings was an assessment that “the use of extreme violence in the pursuit of the perfect Islamic society” has “never been institutionally disowned” by the movement.
The prime minister at the time, David Cameron, who commissioned the report, said membership or association with the Muslim Brotherhood was a "possible indicator of extremism" but it would not be banned.
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What vitamins do we know are beneficial for living in the UAE
Vitamin D: Highly relevant in the UAE due to limited sun exposure; supports bone health, immunity and mood.
Vitamin B12: Important for nerve health and energy production, especially for vegetarians, vegans and individuals with absorption issues.
Iron: Useful only when deficiency or anaemia is confirmed; helps reduce fatigue and support immunity.
Omega-3 (EPA/DHA): Supports heart health and reduces inflammation, especially for those who consume little fish.
Sunday's games
All times UAE:
Tottenham Hotspur v Crystal Palace, 4pm
Manchester City v Arsenal, 6.15pm
Everton v Watford, 8.30pm
Chelsea v Manchester United, 8.30pm
Polarised public
31% in UK say BBC is biased to left-wing views
19% in UK say BBC is biased to right-wing views
19% in UK say BBC is not biased at all
Source: YouGov
UAE currency: the story behind the money in your pockets
The specs: 2018 Volkswagen Teramont
Price, base / as tested Dh137,000 / Dh189,950
Engine 3.6-litre V6
Gearbox Eight-speed automatic
Power 280hp @ 6,200rpm
Torque 360Nm @ 2,750rpm
Fuel economy, combined 11.7L / 100km