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.

In the Closet of the Vatican claims Colombian Cardinal Alfonso Lopez Trujillo was was an arch-defender of the church's teaching on homosexuality while using male prostitutes

He found that a number of clerics spoke of an unspoken code of the 'closet', with one rule of thumb being that the more homophobic they were, the more likely they were gay.

The author, a former adviser to the French government, claims the late Alfonso López Trujillo - a Colombian cardinal who held senior roles in the Vatican - was an arch-defender of the church's teaching on homosexuality and contraception while using male prostitutes, according to Catholic website the Tablet.

The book is a 'startling account of corruption and hypocrisy at the heart of the Vatican', according to British publisher Bloomsbury.

In its marketing material, Bloomsbury claims the book 'reveals secrets' about celibacy, misogyny and plots against Pope Francis.

But critics of the book said 'it is not always easy to tell when Martel is trafficking in fact, rumour, eyewitness accounts or hearsay,' according to the Tablet.

Publisher Bloomsbury claims the book 'reveals secrets' about celibacy, misogyny and plots against Pope Francis

It's due to be released in eight languages across 20 countries next Wednesday, on the day that Pope Francis' holds a summit at the Vatican on sexual abuse attended by bishops from all over the world.

Martel, a former adviser to the French government who is openly gay, claims he spoke with 41 cardinals, 52 bishops and monsignors, 45 papal ambassadors or diplomatic officials, 11 Swiss guards and more than 200 priests and seminarians as research for the book.

The author spent around 200 weeks staying in residences inside the Vatican in Rome, according to the Tablet.

Although the book doesn't conflate homosexuality with the child sex abuse in the church, Martel claims gay priests do not want to report abuse on children in case their on homosexuality is exposed - according to sources familiar with the book's contents.