Kate Middleton has been confirmed in the Church of England at a private ceremony carried out by the Bishop of London, the Rt Rev Richard Chartres.
Miss Middleton, 29, was baptised when she was five months old.
The ceremony, which marks the point where baptised Christians make a firm commitment to their faith, was held on 10 March.
Members of the Church of England usually only receive Holy Communion after they have been confirmed.
Bishop Chartres will also give the address at Kate and Prince William's marriage ceremony on 29 April.
A St James's Palace spokesman said: "Catherine Middleton was confirmed by the Bishop of London at a private service at St James's Palace attended by her family and Prince William.
"Miss Middleton, who was already baptised, decided to be confirmed as part of her marriage preparations."
The Book of Common Prayer requires that "there shall none be admitted to the Holy Communion, until such time as he be confirmed, or be ready and desirous to be confirmed."
William will become Supreme Governor of the Church of England when he succeeds to the throne.
He was also confirmed by the Bishop of London, in a ceremony held at St George's Chapel, Windsor Castle, in March 1997.
It was one of the last times the Prince and Princess of Wales were seen together in public before Princess Diana was killed in a car crash in August that year.