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Britain's Queen Elizabeth is marking her 90th birthday this Thursday. As part of the April 21st celebrations, the Queen is unveiling a plaque marking the start of a four-mile "Queen's Walkway" at the foot of Castle Hill in Windsor.

Pageantmaster Bruno Peek (L) hands Britain's Queen Elizabeth II (L) a torch for her to light the first beacon in honour of her 90th birthday in Windsor on April 21, 2016 with Britain's Prince Charles, Prince of Wales (R) following up. The Queen lit the principal beacon which would set in train a series of beacons across the country and worldwide to mark the Queen's 90th birthday.

She is also taking part in a walkabout with her husband, Prince Philip, to greet members of the public.

The Queen has already taken part in a pair of public events at Windsor in advance of today's celebrations, officially opening a bandstand and touring a mail delivery office to mark the 500th anniversary of the Royal Mail.

Royal watchers say despite her advancing years, Elizabeth II is showing no signs of slowing down.

Royal author Hugo Vickers.

"If she spends a little bit more time in Windsor Castle, so be it, but I can assure you, that she will continue to be a, you know, very active head of state and do as much as she possibly can."

Buckingham Palace has also released 3 new portraits of the Queen, taken by renowned photographer Annie Leibovitz, to mark her 90th birthday.

A new sheet of First Class postage stamps featuring the Queen and her three heirs to the British throne are also being released.