The former pope Benedict XVI has spoken out against allowing married men to become priests, in a move that could jeopardise a potential plan by Pope Francis to change the centuries-old requirement in areas of the Amazon.

Benedict, who implied upon his resignation in 2013 that he would not interfere with the work of his successor, has defended clerical celibacy in an explosive book written with the outspoken conservative cardinal Robert Sarah.

In extracts of the book published by France’s Le Figaro newspaper on Sunday, the authors urged the Catholic church not to be swayed by “bad pleas, theatrics, diabolical lies and fashionable errors that want to devalue priestly celibacy”.