Retired Pope Benedict XVI has reaffirmed his support for priests remaining celibate as his successor looks at potentially relaxing a ban on ordaining married men.

Benedict revealed his views in a book co-authored with Guinean Cardinal Robert Sarah and could be seen as an attempt to sway the thinking of the incumbent Pope Francis.

It comes amid discussion of the issue of allowing married men to become priests in order to address a shortage in the Amazon, where the faithful can go months without having a Mass.

Benedict's intervention is extraordinary, given he had promised to remain "hidden from the world" when he retired in 2013 and pledged his obedience to the new pope.

He has largely held to that pledge, though he penned an essay last year on the sexual abuse scandal that blamed the crisis on the sexual revolution of the 1960s.

In the book titled From the Depths of Our Hearts: Priesthood, Celibacy and the Crisis of the Catholic Church, he writes that marriage requires man to give himself totally to his family.

He said: "Since serving the Lord likewise requires the total gift of a man, it does not seem possible to carry on the two vocations simultaneously.

"Thus, the ability to renounce marriage so as to place oneself totally at the Lord's disposition became a criterion for priestly ministry."

His reaffirmation of priestly celibacy, however, gets to the heart of a fraught policy issue that Francis is expected to weigh in on.

The implications for such an intervention are grave, and are likely to fuel renewed anxiety about the unprecedented situation of two popes, one retired and one reigning, living side by side in the Vatican gardens.