Crowds of cheering people lined the streets in Windsor as the monarch took part in a walkabout, and royal gun salutes have been fired from each of the UK’s capital cities.
The Prince of Wales recorded a special radio broadcast for the day, in which he read an edited passage from William Shakespeare’s Henry VIII.
On Twitter, the Queen thanked senders of “#HappyBirthdayYourMajesty” tweets.
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In another tweet, the Queen said: “I send my best wishes to those who are celebrating their 90th birthday… on this shared occasion, I send my warm congratulations to you.”
The Queen, who was accompanied by the Duke of Edinburgh during her tour in Windsor, was presented with a birthday cake at the Guildhall by the Great British Bake Off champion Nadiya Hussain, who had created an orange drizzle cake with a butter cream and marmalade filling.