Eighty per cent of Vatican priests are gay and living in the closet, according to an explosive new book to be published next week. The 570-page expose, titled In the Closet of the Vatican, claims that four in five clerics in the Roman Catholic Church are homosexuals - but aren't necessarily sexually active. French sociologist and journalist Frederic Martel, who spent four years conducting 1,500 interviews for the book, found that some priests maintained discreet long term relationships, while others lived double lives having casual sex with gay partners and using male prostitutes.