A Briton on Friday became the first person in decades to plead guilty to treason, after admitting trying to harm Queen Elizabeth II with a loaded crossbow in Windsor Castle in 2021.

Jaswant Singh Chail, 21, from southern England, was detained on the grounds of the royal residence on Christmas Day while the late queen was there.

He admitted to an armed officer at the scene that he was there "to kill the queen", and pleaded guilty to three charges at a criminal court hearing.

They include a section of the Treason Act, dating back to 1842 that outlaws attempts to "injure the person of Her Majesty Queen Elizabeth II or to alarm Her Majesty".

He also pleaded guilty to making threats to kill, and possessing an offensive weapon.

After his arrest, it emerged that he had stated his intent in a video recorded four days earlier, which he sent to his phone contacts list about 10 minutes before he was apprehended.

He planned the attack as revenge for the 1919 Jallianwala Bagh massacre of Indians by British colonial troops, previous court hearings heard.

Queen Elizabeth passed away peacefully nearly nine months later, on September 8, aged 96, after a year of failing health.

Chail, an unemployed former supermarket worker, had been due to stand trial later this year over the incident at Windsor, west of London.

But appearing at the capital's Central Criminal Court, known as the Old Bailey, via video-link from high-security psychiatric hospital Broadmoor, he admitted all three charges.

He is now due to be sentenced at the same court on March 31, with medical reports ordered before that.

Nick Price, a senior official of the Crown Prosecution Service (CPS), said it was "a serious incident but fortunately a rare one".

"Thankfully police officers intervened and nobody was hurt," he added in a statement.

In the last such case, Briton Marcus Sarjeant was sentenced to five years' imprisonment in 1981 after pleading guilty to firing blank shots at the queen when she was on a horseback parade in central London.

In 1945, William Joyce -- also known as Lord Haw-Haw, who collaborated with Germany during World War II -- was the last person to be convicted under the separate and more serious 1351 Treason Act.

He was sentenced to death and hanged the following year. (AFP)