It's been an open secret for centuries, but the lid may have finally come off the forbidden sex life of Catholic priests.

A new book alleges that thousands of Catholic priests are conducting secret relationships with men or women - many of them long term - in direct contravention of the Vatican's teachings on celibacy, be it either heterosexual and homosexual.

Italian award-winning investigator Carmelo Abbate spent months documenting the hidden world where heterosexual priests father children with women who can never be their wives - and gay priests of many different nationalities conduct affairs or one night stands in Rome.

Abbate's book, entitled 'Sex and the Vatican: A Secret Journey in the Reign of the Chaste,' was published in Italy on Wednesday and two British publishers are reportedly interested in buying the rights.

"The purpose of the book is not to shame Catholic clergy, it is to expose the hypocrisy and double standards of the church," Abbate, an award-winning investigative reporter, told the press.

"There are priests with children, but the kids cannot talk to their fathers in public for fear of their situation being discovered.

"There is a culture of 'omerta' (silence) in which the church pretends not to know about any of this. If the authorities do find out, they just cover it up so as to avoid any scandal."

Abbate used hidden cameras to film three priests attending gay nightclubs in Rome and even having sex, before donning their cassocks the next day and holding church services.

"At the time the church said that these were just isolated incidents. I wanted to explore whether that was true and what I found was that the phenomenon is much, much wider," he said.

Women who become pregnant by priests reportedly told him of being forced to have abortions - or of having to put their children up for adoption to keep their relationships secret.

"For a lot of priests, the church's teachings on sexual relations are a prison," Abbate said.