Prince William has been made a Royal Knight of the Garter, joining the most senior British order of chivalry.
He was officially appointed by the Queen at a service at St George's Chapel in Windsor Castle.
The Order of the Garter - established by Edward III in 1348 - honours those who have contributed to national life or served the Queen.
William becomes a Royal Knight Companion, which recognises his seniority within the Royal Family.
He becomes the eighth member of the Royal Knights and Ladies, joining other members of the Royal Family.
The order also includes seven foreign monarchs, known as Stranger Knights and Ladies.
The Queen chooses the other 24 members, known as Knights and Ladies Companions, without the advice of ministers.
Prince William's girlfriend, Kate Middleton, was among the guests
The Queen and the Prince of Wales are members ex officio - they are part of the order by virtue of their royal titles.
William's father, Prince Charles, was appointed in 1958, the Princess Royal in 1994, and the Duke of York and the Earl of Wessex in 2006.
The Duke of Edinburgh was made a Royal Knight in 1947 - the same year he married Princess Elizabeth.
The numbers of the 24 Knights Companion were depleted recently with the deaths of the conqueror of Mount Everest, Sir Edmund Hillary, and former Prime Minister Sir Edward Heath.
Their places have been taken by Lord Luce, Lord Chamberlain from 2000 to 2006, and Sir Thomas Dunne, who has been Lord-Lieutenant of Hereford since 1977 and chairman of the Lord Lieutenants Association.
Former prime ministers are usually made members of the order soon after their retirement from office.
The order already includes former Prime Ministers Baroness Thatcher and Sir John Major.
New members of the order are traditionally announced on St George's Day but the ceremonies take place in June, on the Monday of Royal Ascot week, known as Garter Day.
Prince William, who will be the 1,000th Knight in the Register, wore a blue velvet cape and black velvet hat with white ostrich plumes for a procession from the castle to the chapel.
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