Traditional gun salutes will mark the Queen's 88th birthday today.
She will be spending the day privately at Windsor Castle, a Buckingham Palace spokeswoman said.
The King's Troop Royal Horse Artillery will fire a 41-gun royal salute from Green Park in central London at midday, using six 13 pounder First World War guns drawn by 71 horses.
An hour later the Honourable Artillery Company will fire a 62 Gun royal salute from Tower Wharf by the Tower of London.
Before the King's Troop arrive in Green Park, the Band of the Honourable Artillery Company and Royal Yeomanry will play a selection of celebratory music close to the firing position.
Princess Elizabeth Alexandra Mary was born at 2.40am on April 21 1926 - the first child of the then Duke and Duchess of York - at 17 Bruton Street, the Mayfair home of her mother's parents, the Earl and Countess of Strathmore.
She is the great-great-grandchild of Queen Victoria, the last female sovereign.
The Queen has two birthdays - her actual one today and her official one in June, which is celebrated with the Trooping the Colour parade.
A portrait of the Queen by the renowned British photographer David Bailey was released yesterday ahead of her 88th birthday.
The photograph of the monarch, smiling, was taken at Buckingham Palace in March and commissioned on behalf of the Government's GREAT Britain campaign.
The Queen is wearing pearls and a dress by Angela Kelly, who has served as personal assistant and senior dresser to her since 2002.