Pope Francis is to grant all Catholic priests the power to forgive women who have had abortions, in a significant softening of the Church's stance on the issue.

In a letter published on Tuesday by the Vatican, Francis said he had "met so many women who bear in their heart the scar of this agonising and painful decision", and described the plight facing women who terminated pregnancies as an "existential and moral ordeal".

The Catholic Church has historically deemed abortion so great a sin that those who receive them or carry them out are automatically excommunicated.

That is not about to change, the Vatican said. But by allowing all priests the power to forgive - which was previously reserved for designated clergy - he has made another move towards a more open and inclusive church.

"This is by no means an attempt to minimize the gravity of this sin but to widen the possibility of showing mercy," Vatican chief spokesman Father Federico Lombardi told reporters.

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The change is to be introduced at the start of the Holy Year, announced by the Pope on the theme of "mercy" and due to run from 8 December to 26 November 2016.

Deputy Vatican spokesman Father Ciro Benedettini said the change would apply only during the Holy Year "for now".