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Three new portraits of Queen Elizabeth taken by American photographer Annie Leibovitz were released on Thursday to mark Britain's longest-reigning monarch's 90th birthday.

Leibovitz, who has photographed Elizabeth before, took the official pictures at Windsor Castle, west of London.

The first in the sequence of relaxed images shows the monarch walking with four of her dogs in the castle grounds.

Queen Elizabeth II poses on the steps of the east terrace with four of her dogs, clockwise from top left, Willow, Vulcan, Candy and Holly in the garden of Windsor Castle. ANNIE LEIBOVITZ / via AFP - Getty Images

In the second picture, top, she sits with her two youngest grandchildren and five great-grandchildren in the Green Drawing Room, part of the castle's semi-State apartments. Youngest great-grandchild Princess Charlotte, the 11-month-old daughter of Prince William and his wife Kate, sits on her lap.

The third portrait shows the queen with her daughter Anne, the Princess Royal, in the castle's White Drawing Room.

Queen Elizabeth poses with her daughter, Princess Anne, in the White Drawing Room at Windsor Castle. Annie Leibovitz / via EPA

Elizabeth was due to celebrate her birthday with a walkabout in Windsor, meeting well-wishers. In London, there were due to be artillery gun salutes at Hyde Park and the Tower of London and parliament was to be lit up red, white and blue.

Born on April 21, 1926, Elizabeth last year surpassed the 23,226-day reign of her great-great-grandmother, Queen Victoria.