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He donned the traditional blue velvet cape and a black ostrich plume-adorned hat to join other Knights of the Garter in a procession to the chapel following a lunch at the castle.
Fellow knights include the Duke of Edinburgh and the Earl of Wessex, Princess Anne and Prince Andrew.
The Queen – who is the only person authorised to grant membership – is known as the Sovereign of the Garter.
She was flanked during the procession by two pages of honour - her great-nephews, Arthur Chatto and the Hon. Charles Armstrong-Jones.
Meanwhile Prince Charles, who walked by William's side, is a Knight Companion of the Garter.
The Knight of the Garter - the most senior British order of chivalry - is traditionally given to those who have held public office, have contributed to national life or who have served the monarch personally.
Founded by Edward III in 1348, there are a maximum of 24 companion knights at any one time and their number is usually made up of senior politicians, members of the royal family and eminent public figures.
New knights are invested in Windsor Castle's Garter Throne Room then 'installed' during a service in St George's Chapel.
The Duchess – then William's girlfriend and still just Kate Middleton - was watching in 2008 when William was officially appointed the 1,000th Royal Knight of the Garter.
Back then Kate was seen giggling with Prince Harry as her future husband filed past dressed in his robes.