Well-wishers have been queuing outside Windsor Castle since Tuesday to get the best spot for the Queen on her birthday walk-about.
Royal enthusiasts Anne and Judy came left Cardiff at 3am, along with their King Charles spaniel named Camilla after the Duchess of Cornwall, to "share her great day".
The sisters told GMB they "admire her stamina" and they "absolutely adore the Queen".
Crowds have begun to line the streets of Windsor as fans wait to see the Queen on her 90th birthday.
The Queen, accompanied by The Duke of Edinburgh, will later unveil a plaque marking The Queen's Walkway at the foot of Castle Hill, before undertaking a walkabout to meet the public and well-wishers.
ITV News Royal Editor Tim Ewart is in Windsor:
The Royal Navy will mark the birthday of Queen Elizabeth II with a 21-gun salute at Fort Blockhouse in Gosport today.
The event is one of many taking place at military establishments across the country. At Portsmouth's Naval base, Royal Navy ships will celebrate with flags displayed along their sides.
"The Royal Navy in Portsmouth always celebrates the Queen's birthday in this way, but this year is obviously extra special as it marks such a significant milestone for Her Majesty."
Her Majesty The Queen is ninety years old today, and Windsor in Berkshire is the focus of the celebrations. Queen Elizabeth II will go on a walkabout around the town, accompanied by her husband, the Duke of Edinburgh.
The Queen will also unveil a plaque marking 'The Queen's Walkway', a 6.3km self-guided walking trail connecting 63 points of significance in the town of Windsor. The Walkway symbolises the moment at which Her Majesty became Britain's longest reigning monarch, after more than 63 years on the 9th September 2015.
Celebrations will also take place in June to mark Her Majesty The Queen's official birthday.
The Queen turns 90 this week, and during her reign of more than 60 years she's travelled to the Meridian region on many occasions.
We're looking back at your memories. Fred went to meet Leslie Smith who was chosen to meet the Queen when he had just started his Royal Navy career at the age of 16.
The Queen's 90th birthday celebrations have begun in the Thames Valley. She appeared in public in Windsor to open a new bandstand and to visit the local Post Office - marking 500 years of the Royal Mail.
While the Queen was there, she saw the new commemorative stamps - which feature portraits from a picture of four generations of the House of Windsor.
Tomorrow, Berkshire will welcome the world's media when she will go on an official walkabout. But, as Divya Kohli reports, Windsor is already in a celebratory mood.
Archive film of the Queen as a child from The Royal Collection Trust.
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