Francis said he had "learned my lesson" after allowing an Italian bishop to not defrock a priest who had been found guilty of acts of abuse, and who then committed similar offences two year later.
"The abuse of a minor, if it is proven, is sufficient for there to be no possibility of appeal. If the proof is there, the punishment is definitive," the pope said in improvised comments to his child abuse advisory panel.
"And as for requests for papal pardons, I will not sign anything for these crimes."
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Francis also acknowledged that the Church had been slow to wake up to the scale of the problem of clerical abuse, which has done enormous damage to its standing in many countries.
"The means of resolving the problem are also arriving a bit late," he said. "That is the reality, the old practice of moving (paedophile priests) from one diocese to the other put people's conscience to sleep."
Francis has repeatedly vowed to rid the Church of the scourge of paedophilia through a zero-tolerance approach which his predecessors proved incapable of implementing.
But his credibility on the issue has been hit by the resignation of two members of his advisory panel over opposition to changes from within the Vatican hierarchy.
Victims' organizations also maintain that the Church remains reluctant to hand paedophile priests over to criminal justice authorities.