Buckingham Palace has shared a photo of the Queen as a baby as she turns 91 on Friday, 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 for her 90th birthday.
Royal gun salutes were performed across London and in other parts of the UK.
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.
While her 91st was a more private affair, gun salutes took place in London – at midday, there was a 41-gun salute in Hyde Park and 21-gun salute in Windsor Great Park, ahead of a 62-gun salute at the Tower of London at 13:00 BST.
There was also a 21-gun royal salute fired from Edinburgh Castle.
Bells pealed at Westminster Abbey to mark the occasion and Irish Guards performed a rendition of Happy Birthday to You outside Buckingham Palace.
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. They also released a picture of the Queen when she was 21 as she received the Freedom of the City of London – the first significant London ceremony she attended unaccompanied.
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.
Canadian Prime Minister Justin Trudeau tweeted a birthday message, wishing the Queen “a happy 91st birthday and continued health and happiness in the years to come”, while Foreign Secretary Boris Johnson tweeted: “Hip hip hooray for The Queen and MANY CONGRATS to Her Majesty”.
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 election announced for Thursday 8 June will be the 17th general election of the Queen’s reign.
Gifts fit for a Queen
While it is not known what the Queen has received for her 91st birthday, she is known to favour practical gifts over more extravagant presents.
Her children have given her items including a casserole dish and apron over the years, while the Duchess of Cambridge made her some chutney for Christmas – following her own grandmother’s recipe.
Speaking on BBC Radio 1 on Friday, the Duke of Cambridge admitted it was “quite hard to know what to get” his grandmother “that she hasn’t already got”.
He said now “the great-grandchildren can make stuff”, which “goes down really well”, adding that three-year-old George is “very good at arts and craft”.
Many of her favourite presents over the years have reflected her love of animals.
When she was four, the Queen’s grandfather King George V gave her her first horse, a Shetland pony called Peggy.
And when she was 18, she was given a corgi called Susan, sparking a lifelong affection for the dog breed. She has now owned more than 30 corgis.