Queen Elizabeth, 92, and her 97-year-old husband Prince Philip attended the christenings of Louis' older siblings Prince George and Princess Charlotte.
Kate's parents Carole and Michael Middleton, her brother James and her pregnant sister Pippa were also present.
According to reports, Queen Elizabeth II then asked her personal dresser Angela Kelly for a replica of the gown to be made. The robe is a family heirloom that dates all the way back to 1841, when Queen Victoria's oldest daughter wore it for her christening.
The Archbishop of Canterbury Justin Welby welcomed the royals at the entrance to the chapel.
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"I hope he stays like this", she added. Nicholas van Cutsem, Guy Pelly, Harry Aubrey-Fletcher, Lady Laura Meade, Mrs Robert Carter and Lucy Middleton all agreed to the honor prior to the ceremony.
Guests were served slices of christening cake, which is a tier taken from The Duke and Duchess of Cambridge's wedding cake. As is tradition, the holy water used in the ceremony will be drawn from the River Jordan.
Of baby Louis, she gave birth at St. Mary's hospital in London, where was born the Prince George and Princess Charlotte.
George, 4 and Charlotte, 3, were there for their brother's big day.
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