Buckingham Palace has shared a photo of the Queen as a baby to mark her 91st birthday, amid online tributes and military salutes to mark the occasion.
The monarch is spending the day in private at Windsor Castle, in contrast with last year's national festivities.
Guns will fire at London's Hyde Park at 12:00 BST to mark the day.
The Queen, who reached her 65-year Sapphire Jubilee in February, is Britain's longest-reigning monarch.
Last year, more than 900 beacons were lit up across the UK and overseas as part of her 90th birthday celebrations. Crowds lined the streets in Windsor, church services were held in her honour, and the Queen took part in a walkabout at Windsor.
Buckingham Palace tweeted a picture of the monarch as a baby on her christening day, where she is held by her mother Elizabeth, Duchess of York.
Clarence House, the Prince of Wales and Duchess of Cornwall's official residence, shared a picture of the Queen and a young Prince Charles from 1952.
Well-wishers used the hashtag #HappyBirthdayHerMajesty to tweet their own birthday messages.
The Royal Albert Hall said: "Happy birthday to our patron, Queen Elizabeth!" and a Scout group in Bristol tweeted: "#HappyBirthdayHerMajesty... And wish you a prosperous day!"
The Queen also celebrates an "official birthday" which falls on the second Saturday in June.
It will be marked with a parade in London known as the Trooping the Colour ceremony - this year's takes place the week later, on 17 June.
Two days earlier, the snap general election announced for Thursday 8 June will be the 17th of the Queen's reign.
The Queen typically spends her actual birthday privately, but the occasion is marked publicly by gun salutes in central London at midday.
There will be a 41-gun salute in Hyde Park, a 21-gun salute in Windsor Great Park and a 62-gun salute at the Tower of London.Image copyright Getty Images Image caption