Retired former pope Benedict XVI sought to calm a dispute within the Vatican on Tuesday, claiming his name was erroneously attributed as co-author to a new book with a conservative cardinal.

Publication of extracts from the book, "The Depth of our Hearts," by French newspaper Le Figaro on Sunday set off a storm within the Vatican, with some questioning why the 92-year-old Benedict would come out of retirement to co-author a book with ultra-conservative Cardinal Robert Sarah.

On Tuesday, the ex-pontiff's private secretary, Georg Gaenswein, clarified that although Benedict had given Sarah passages he had written to use as the cardinal saw fit, "he hadn't approved any plans for a double signature book nor had he seen and authorised the cover."

"It was a misunderstanding, without questioning the good faith of Cardinal Sarah," Gaenswein told Italian newswire ANSA.

In the book, which will hit bookshelves in France on Wednesday, Benedict is quoted as saying he "cannot keep silent" about the divisive issue of whether or not to allow married men to become Catholic priests -- and coming down firmly against it.

Pope Francis is currently considering allowing it in remote locations, such as the Amazon, where communities seldom have Mass due to a lack of priests. He is expected to publish his decision in the coming weeks.

Vatican experts had expressed astonishment that the retired pope would speak out in a public way on such a sensitive topic, and sources late Monday said Benedict had never authorised the book.

"Benedict XVI... never saw, nor authorised the cover, nor did he authorise the publication of a co-authored book," a Vatican source "very close" to the former pope told Elisabetta Pique, Vatican correspondent for Argentina's La Nacion.

Cardinal Sarah took to Twitter on Tuesday to defend himself, saying Benedict knew their collaboration would be a book, and that they had sent proofs back and forth for corrections.

The book highlighting Benedict's position on celibacy in the priesthood came as Pope Francis considers possible exceptions to the rule
The book highlighting Benedict's position on celibacy in the priesthood came as Pope Francis considers possible exceptions to the rule VATICAN MEDIA / Handout

He later issued a statement affirming his "affection" for Benedict and "obedience" to Pope Francis, and appeared to have been forced to row back on the issue.

"Considering the polemics caused... it has been decided that the author of the book will be for future publications: Card Sarah, with the contribution of Benedict XVI," he said in a tweet.

"However, the complete text remains absolutely unchanged," he insisted.

The book features an essay by Benedict and another by Sarah, with a co-authored introduction and conclusion. It has both the former pope and cardinal on the cover.

Vatican expert Christopher Lamb said the controversy was not about the book's content, but rather over "the use of a retired pope's authority to make the point."

"No-one is doubting that Benedict agrees with the premise of the book," Lamb said on Twitter.

Nicolas Seneze of the French Catholic daily La Croix reported a flurry of exchanges Monday between Benedict's abode and Francis's, "where the danger of a book that erects the pope emeritus as a parallel magisterium (Catholic authority) was clearly understood".

Italian daily Repubblica also weighed in on the controversy.

At the former monastery in the Vatican gardens, Benedict's home since becoming the first pontiff to resign in almost 600 years, the fear was "that the emeritus pope has been used without his knowledge", it reported.

It warned of "the real risk that there are those... who use Benedict to advance their own battles".