This year, the monarch sees her birthday coincide with Easter Sunday, the most important day in the Christian calendar.

Elizabeth II, who has a strong faith, usually celebrates Easter at Windsor Castle by joining members of her family at a service in St George's Chapel.

At Buckingham Palace, the Band of the Irish Guards will perform a rendition of Happy Birthday in her honour on the forecourt as part of the changing of the guard ceremony.

Gun salutes will also be performed at Hyde Park and the Tower of London on Monday in the Queen's honour.

April 21 - the day the Queen was actually born - is the first of the monarch's two birthdays.

Her official birthday is celebrated in most of Australia on the second Monday in June.

The Queen traditionally spends a month-long residence at Windsor Castle, known as Easter Court, each year.

With the death of 88-year-old King Bhumibol Adulyadej of Thailand in 2016, the Queen now holds the record for the world's longest reigning living monarch.

Australian Associated Press