The Catholic Church is "nowhere close" to enacting the reforms needed to stop the "epidemic" of sex abuse by predatory priests and bishops against children, campaigners warned on Tuesday.

Pope Francis is "in retreat" from any meaningful effort to bring abusers to justice, said Bishop Accountability, a leading pressure group.

The scathing criticism comes as the Vatican admits it has secret guidelines on how to deal with priests who break their celibacy vows and sire children.

Nearly 200 archbishops, bishops and other senior officials are to join the Pope at the Vatican for an unprecedented, four-day conference on combating the sexual abuse of minors by clergy.

Like his predecessors, the Pope has fostered "a culture of plausible deniability" in which allegations against priests are lost, not scrutinised properly, or buried in bureaucracy, campaigners said.

The Catholic Church persists in regarding the sexual abuse of children as a sin, to be dealt with internally, rather than as a serious crime that requires the intervention of the police, said Phil Saviano, a high-profile survivor of sex abuse.

Molested by a priest in Massachusetts when he was 12 years old, his ordeal was told in the Oscar-winning film Spotlight, based on a Boston Globe investigation into widespread sex abuse by clergy.