Pope Emeritus Benedict XVI arrives in St. Peter's Square at the Vatican in 2014. File picture: Andrew Medichini/AP

Vatican City - A top cardinal denied Tuesday that a controversial book he co-authored with Benedict XVI is being published without the consent of the retired pope.

In the book, Benedict argues against a relaxation of the celibacy rule for priests, wading into an issue on which his successor Pope Francis is due to make a decision within a few weeks.

Several commentators have seen this as an unwelcome interference, and a serious breach of Benedict's promise to remain "hidden to the world" following his retirement in 2013.

But to insinuate that the former pope "had not been informed" about the book coming out "is extremely despicable," Cardinal Robert Sarah said in a statement.

"I sincerely forgive all those who are slandering me or who want to put me against Pope Francis," he added, renewing his affection for Benedict and "absolute filial obedience to Pope Francis."

Cardinal Robert Sarah, of Guinea, walks in St. Peter's Square after attending a cardinals' meeting in 2013. File picture: Andrew Medichini/AP

Sarah responded to Italian newspaper reports quoting an unnamed source from Benedict's entourage as claiming that the former pope never agreed to publish a book with Sarah.

The reports came after several Vatican commentators suggested that Benedict, who is 92 and very frail, had been manipulated by others into releasing an apparent anti-Francis document.

The book is called "From the Depths of Our Hearts: Priesthood, Celibacy and the Crisis of the Catholic Church" and comes out in France on Wednesday. An English version is due out on February 20.

The celibacy question has moved to the forefront after a recent recommendation that would allow married men to become priests in the Amazon area, because of the difficulty in finding people willing to serve there.