A former top Vatican official accused Pope Francis of having known of allegations of sex abuse by a prominent U.S. cardinal for five years before accepting his resignation last month and he called on the pontiff to resign.

In an 11-page letter given to conservative Roman Catholic media outlets during the Pope's visit to Ireland, Archbishop Carlo Maria Vigano said he had told Francis in 2013 that Cardinal Theodore McCarrick had faced extensive accusations of sexually abusing lower-ranking seminarians and priests.

Vatican officials declined immediate comment on the letter on Sunday.

McCarrick became the first Cardinal in living memory to resign his position in the Church leadership after a review concluded that claims he had sexually abused a 16-year-old boy were credible.

He was one of the highest-ranking church officials accused of sex abuse in a scandal that has rocked the 1.2 billion-member faith since reports of priests abusing children and bishops covering up for them were first reported by the Boston Globe in 2002.

Since then, patterns of widespread abuse of children have been reported across the United States and Europe, in Chile and Australia, undercutting the Church's moral authority and taking a toll on its membership and coffers.

Vigano said in the letter reported by the conservative U.S. National Catholic Register that he had told Francis of allegations against McCarrick in June 2013 shortly after his election as Pope by his fellow cardinals.

"He knew from at least June 23, 2013, that McCarrick was a serial predator," Vigano, who served as the Vatican's ambassador to the United States at the time.

"Pope Francis must be the first to set a good example for cardinals and bishops who covered up McCarrick's abuses and resign along with all of them."

Vigano's letter railed against "homosexual networks present in the Church" -- the word "homosexual" appears 18 times, while the word "child" appears only twice, in both cases in the titles of Church documents Vigano sites.

Francis vowed on Saturday to end the sexual exploitation of children by clergy during a highly-charged visit to once deeply Catholic Ireland and, according to victims, said the corruption and cover-up of abuse amounted to human excrement.