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 BBC reports.
The monarch is spending the day in private at Windsor Castle, in contrast with last year’s national festivities.
Guns fired 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.
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.