"For the first time in history we are no longer at the top. The Muslims have overtaken us," the media quoted Monsignor Vittorio Formenti, who compiled the Vatican's 2008 yearbook of statistics, as saying.
According to figures, Muslims make up 19.2 per cent of the world's population while Catholics 17.4 per cent.
However, Monsignor Formenti said that if all Christian groups were considered, including Orthodox churches, Anglicans and Protestants, Christians made up 33 per cent of the world's population - or about two billion. The data referred to 2006.
The Vatican recently put the number of Roman Catholics in the world at 1.13 billion. The figure for Muslims, on the other hand, is estimated at around 1.3 billion.
According to Monsignor Formenti, while the number of Catholics as a proportion of the population is fairly stable, the percentage of Muslims has been growing because of higher birth rates.
He said the data on Muslim populations had been compiled by individual countries and then released by the United Nations, adding that the Vatican could only corroborate for its own statistics.