The Queen celebrated her 77th birthday with a horse ride in the grounds of Windsor Castle.
The Queen was riding with a groom
She was waved off by her granddaughter Zara Phillips and accompanied by a groom.
At 1200 BST, 21-gun
salutes were fired by the King's Troop, Royal Horse Artillery, at London's Hyde
Park, and 105 Regiment Royal Artillery (Volunteers) at Stirling and Edinburgh Castles.
A Buckingham Palace spokeswoman said the Queen would spend the day with the Duke of Edinburgh and other family and friends at Windsor Castle.
On Sunday the Queen attended an Easter service at St George's Chapel in Windsor.
She was joined by the Duke of Edinburgh, the Princess Royal, the Duke of York and
the Earl and Countess of Wessex.
A bunch of yellow tulips was handed to her after the service by six-year-old Amie Scott, from St Austell in Cornwall.
Gun salutes marked the occasion
Monday is the Queen's real birthday - as opposed to her official birthday, which is celebrated in June.
The Queen was born at 0240 BST on 21 April, 1926, at 17 Bruton Street in London.
The day will also see the Queen's Award for Enterprise handed out to a total of 123 companies.
A purveyor of lentils and a frozen marine worms company feature among the recipients this year.
The successful companies are nominated in three categories - international trade, sustainable development and innovation.
The Queen's Awards have been running in some form since 1966.