Pope Francis has extended indefinitely to all Roman Catholic priests the power to forgive abortion, a right previously reserved for bishops or special confessors.

Francis, who has made a more inclusive and forgiving Roman Catholic Church a characteristic of his papacy, made the announcement in a document known as an "apostolic letter" after Sunday's close of the Church's "Holy Year of Mercy".

He said he wanted to "restate as firmly as I can that abortion is a grave sin, since it puts an end to an innocent life" but "there is no sin that God's mercy cannot reach and wipe away when it finds a repentant heart seeking to be reconciled with (God)".

Francis had already temporarily granted the power to all priests to give what is known as "sacramental absolution" for abortion during the Holy Year, from December 8 to November 20, but the solemn tone of his words in Monday's letter suggested that change would last for at least the rest of his papacy.

"I henceforth grant to all priests, in virtue of their ministry, the faculty to absolve those who have committed the sin of procured abortion. The provision I had made in this regard, limited to the duration of the Extraordinary Holy Year, is hereby extended ...," he said.

In Roman Catholic teaching, abortion is such a serious sin that those who procure or perform it incur an automatic excommunication, until it is absolved in confession.

In the past, only a bishop or a designated chief confessor of a diocese could grant absolution for an abortion.

Although bishops in some dioceses in developed countries such as the United States and Britain had already delegated this authority to parish priests, the old practice was still in effect in most of the world.

In a document last year, Francis described the "existential and moral ordeal" faced by women who have terminated pregnancies and said he had "met so many women who bear in their heart the scar of this agonising and painful decision".

In the past, only a bishop or a designated chief confessor of a diocese could grant absolution for an abortion. Photo: Shutterstock