A man arrested with a crossbow at the Queen’s Windsor Castle home has been charged with treason for having a weapon with intent to “alarm Her Majesty”, police and prosecutors say.

Jaswant Singh Chail, 20, from Southampton in southern England, was arrested on Christmas Day last year while carrying a crossbow in the grounds of the castle west of London.

Windsor is the usual home of the 96-year-old monarch and she was in residence at the time. Her son and heir Prince Charles, his wife Camilla and other close family were also there.

Following an investigation by counter-terrorism police, Mr Chail was charged with making threats to kill, possession of an offensive weapon and an offence under section two of the Treason Act 1842.

It details punishment for “discharging or aiming fire-arms, or throwing or using any offensive matter or weapon, with intent to injure or alarm Her Majesty”.

He remains in custody, and will face London’s Westminster Magistrates’ Court on August 17.

“The Crown Prosecution Service reminds all concerned that criminal proceedings against Mr Chail are active and that he has the right to a fair trial,” CPS special crime and counter terrorism division chief Nick Price said.

Police said Mr Chail had not broken into any buildings.

Security breaches at royal residences are rare.

The most serious in the Queen’s reign was in 1982, when an intruder climbed a wall to enter Buckingham Palace, her London home, and made his way to her bedroom.

Michael Fagan later told The Sun how he pulled back the bed curtains to discover the Queen asleep in her bed.

“They say she must have been frightened. I didn’t frighten her too much but I was quite shocked,” he said.

“She used a phone on the bedside table to call security but when nobody came she got out of bed.”

The monarch summoned a footman – who quickly called the police. Mr Fagan was eventually charged with burglary, due to an earlier break-in at the palace, although he was acquitted by a jury.

In 2003, Aaron Barschak, who called himself “the comedy terrorist,” evaded security at Windsor wearing a pink dress and an Osama bin Laden-styled beard to gatecrash the 21st birthday party of Charles’s elder son Prince William, the second-in-line for the throne.

The last person convicted under the 1842 Treason Act was Marcus Serjeant, a 17-year-old who fired six blank shots at the Queen during the Trooping the Colour ceremony in 1981.

The last person convicted under the more serious mediaeval 1351 Treason Act was William Joyce. He was a propagandist for Nazi Germany nicknamed Lord Haw Haw who broadcast to Britain during World War II. He was hanged in 1946.

Fortunately for Mr Chail, he will not face any likely execution, not least because Britain formally abolished that penalty for high treason in 1998. The maximum sentence under section two of the 1842 Act is seven years’ jail.

-with AAP