A strongly worded attack by Pope Emeritus Benedict XVI on the prospect of allowing some married men to become priests has exposed the perils of having a “parallel papacy” within the Vatican, insiders said on Monday.

In a move which has shaken the Catholic Church, Benedict has co-authored a book in which he says he can no longer stay silent on the idea of addressing the chronic shortage of priests in regions such as the Amazon by letting married men become ordained.

The book, in which 92-year-old Benedict staunchly defends the principle of celibacy for priests, has been interpreted as a bold attack on Pope Francis and the latest salvo in the ongoing war between traditionalists and reformers within the Church.

There were fears when Benedict resigned seven years ago that by assuming the title of emeritus pope and choosing to live within the Vatican, just a few hundred yards from his successor, that he could become the focus for enemies of Francis.

Those fears appear to have been realised, despite Benedict initially promising that he would stay “hidden from the world” after his historic resignation.