The new Bishop of Rome, also known as Pope Francis, will be installed on March 19.
The Duke of Gloucester will represent Queen Elizabeth II at the ceremony. He will be accompanied by his wife, the Duchess of Gloucester.
The King and Queen of the Belgians will be attending as well. The Prince of Orange and the Argentine-born Princess Maxima of the Netherlands will represent Queen Beatrix.
The Prince and Princess of Asturias will represent King Juan Carlos, who is recovering from surgery.
The Grand Duke and Grand Duchess of Luxembourg and Prince Félix of Luxembourg will attend the ceremony. Prince Félix attends Pontifical Athenaeum Regina Apostolorum university in Rome.
The Prince and Princess of Monaco are also now on the list of royal attendees.
Vice President Joseph Biden, a Roman Catholic, will head the United States delegation to the Installation.
Someone wondered if Lord and Lady Nicholas Windsor will be at the ceremony. They are not members of the Royal Family, and do not carry out official engagements. They live in Rome, and have strong ties to the Vatican, so it would not be a surprise if they attend the service as private citizens with connections. In October 2011, Lord Nicholas was appointed as Chairman of the Rome-based Dignitatis Humanae Institute. He served in this position until February of this year.