Prince William and Kate Middleton, during a photocall in the State Apartments of St James's Palace, London to mark their engagement (John Stillwell/PA Wire)

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Prince William is to marry his long-term girlfriend Kate Middleton on Friday 29 April 2011. The ceremony will take place in Westminster Abbey. Prime Minister David Cameron said it would be "a happy and momentous occasion" and would be marked by a public holiday. In this picture the couple pose during a photocall at the State Apartments of St James's Palace in London to mark their engagement. Click through the images below for more.