It is one of three images taken by celebrity photographer Annie Leibovitz.

Celebrations get under way later as the Queen takes part in a walkabout in Windsor and lights a symbolic beacon.

Prime Minister David Cameron said in a video tribute that the Queen had been "steadfast - a rock of strength for our nation, for our Commonwealth".

Gun salutes take place around the UK at noon, while the prime minister is expected to lead tributes to Britain's longest serving monarch in the Commons.

To coincide with the Queen's birthday, the largest exhibition of the Queen's clothes and accessories ever shown in Scotland will open at the Palace of Holyroodhouse later.

The display has been selected to cover the Queen's life and reign and includes evening and day wear.

On Friday, US President Barack Obama and First Lady Michelle Obama will be joining the Queen for lunch at Windsor Castle.

The Queen each year celebrates two birthdays, with her actual birthday on 21 April and her official birthday on a Saturday in June.

Celebrations of her official birthday this year take place from 10-12 June.

( Extracts from BBC)