The Queen celebrates her 87th birthday today.
A spokesperson for Buckingham Palace said Queen Elizabeth II will mark the occassion privately, while the milestone will be marked across the UK with traditional gun salutes fired in celebration.
It is thought the Queen and Duke of Edinburgh are staying at Windsor Castle, said to be their favourite home.
Queen Elizabeth II marks two birthdays each year - her actual birthday celebrated on April 21 and an official birthday in June, marked with the Trooping the Colour parade.
This summer the monarch will also mark the 60th anniversary of her coronation, staged at Westminster Abbey on June 2 1953.
A royal history
Princess Elizabeth Alexandra Mary was born at 2.40am on April 21 1926 at 17 Bruton Street, the Mayfair home of her mother's parents, the Earl and Countess of Strathmore.
She was the first child of the Duke and Duchess of York, and the great-great-grandchild of Queen Victoria, the previous female sovereign.
Queen Elizabeth II, who is Head of State, the Armed Forces, the Commonwealth and the Church of England, has been married to the Duke of Edinburgh for more than 65 years.
She has four children and is grandmother to eight grandchildren. Her third great grandchild and a future sovereign is expected to be born in mid-July to the Duke and Duchess of Cambridge.