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Royal watchers had expected the groom, a helicopter rescue pilot, to wear his navy blue air force uniform to take his vows at Westminster Abbey.
But palace officials said William has opted to wear the scarlet tunic of an Irish Guards officer when he weds Middleton - a tribute to his honorary rank of colonel in the regiment.
William's choice of ceremonial military dress sends a strong signal of his support for the armed forces, reinforcing his preferred image as a dedicated military man and distancing him from past characterisations as a club-hopping party boy.
While the groom could have chosen the morning suit with tails, William opted to take his father's lead in wearing a ceremonial military uniform. Prince Charles wed William's mother, Lady Diana Spencer in 1981, wearing full naval dress.
Still, the young prince's choice is a slight break from recent tradition - the last major royal wedding to feature a groom marrying in red was when Captain Mark Phillips, then an officer with the 1st Queen's Dragoon Guards, married Charles's sister, Princess Anne, in 1973.
William will sport his Royal Air Force "wings" as a tribute to his career as a search and rescue helicopter pilot. He will also wear a Garter sash and star and a Golden Jubilee medal, topped off with a forage cap bearing the Irish Guards insignia and motto, "Quis Separabit?" - "Who shall separate us?"
Other members of the royal family will also be in military dress when William and Middleton take their vows.
Best-man Prince Harry is due to wear the uniform of his new rank as Captain in the Household Cavalry's Blues and Royals. A trainee Apache helicopter pilot, Harry will wear the "wings" of the Army Air Corps and his Golden Jubilee and Afghanistan Campaign medals.
Charles, who has held the title of Admiral of the Royal Navy since 2006, will wear his blue Royal Navy number one ceremonial dress, embellished with a blue sash. He will wear numerous military decorations and carry his Royal Naval sword.
Charles's father, Prince Philip, will wear his Grenadier Guards uniform - a scarlet tunic and dark blue trousers with a red stripe.