They include a section of the 180-year-old Treason Act, admitting an “intent… to injure the person of Her Majesty Queen Elizabeth II, or to alarm Her Majesty”.
He had also been charged with making a threat to kill the Queen and having a loaded crossbow, an offensive weapon, in a public place.
Chail, from southern England, allegedly told a protection officer when he was apprehended that he was “here to kill the Queen”.
He planned an attack as revenge for the 1919 Jallianwala Bagh massacre of Indians by British colonial troops, previous court hearings heard.
He had sent out a video saying he would assassinate the monarch, who passed away peacefully on September 8 aged 96 after a year of failing health.
The 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.
The incident happened as the Queen spent Christmas Day that year at Windsor Castle with her eldest son and heir, Prince Charles, and his wife Camilla.
It is understood Chail had scaled the perimeter of the grounds with a nylon rope ladder some two hours before he was detained.
He was said to be wearing a hood and mask and was carrying a crossbow loaded with a bolt, with the safety catch off and ready to fire.
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 monarch when she was on parade.
However, 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.