Pope Francis blows out the candles for his birthday during an audience for children and families of the Santa Marta dispensary on Sunday at the Paul VI Audience Hall in the Vatican. Pope Francis turned 83 on Tuesday. Photo: AFP

Former pope Benedict XVI has publicly urged his successor Pope Francis not to open the Catholic priesthood up to married men, in a plea that Sunday stunned Vatican experts.

The ex-pontiff, who retired in 2013, issued the defense of clerical celibacy in a book written with arch-conservative Cardinal Robert Sarah, extracts of which were published in exclusive by France's Le Figaro.

"I cannot keep silent!" Benedict wrote in the book, which follows an extraordinary meeting of bishops from the Amazonian at the Vatican last year that recommended the ordination of married men in certain circumstances.

The pope emeritus, 92, and Sarah from Guinea weighed in on the controversial question of whether or not to allow "viri probati" - married "men of proven virtue" - to join the priesthood.

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

The pair asked the whole Church not to be "swayed" by "bad pleas, theatrics, diabolical lies, fashionable errors that want to devalue priestly celibacy."

They warned of priests "confused by the incessant questioning of their consecrated celibacy."

"The conjugal state concerns man in his totality, and since the service of the Lord also requires the total gift of man, it does not seem possible to realize the two vocations simultaneously," Benedict wrote.

Sarah insisted that while celibacy can be "a trial" it is also "a liberation."

Benedict, who was the first pontiff to resign in almost 600 years, at first withdrew to a life of quiet contemplation in the Vatican, but has increasingly begun to speak out on key Catholic issues.

He and Sarah insisted their plea was not a "political maneuver" or "power game."

But Vatican experts expressed astonishment that the retired pope would speak out on such a sensitive topic.

AFP