Pope Francis has said the Catholic Church will keep its ban on women becoming priests forever.

The Pontiff made the remarks on Tuesday after a reporter asked if he thought women would ever be ordained as ministers in the future.

He was speaking to journalists while flying back to Rome from Sweden, where he had been welcomed by the female head of the Lutheran Church.

"St Pope John Paul II had the last clear word on this and it stands, this stands," the Pope said.

He was referring to a 1994 document by Pope John Paul that closed the door on a female priesthood. The Vatican says this teaching is an infallible part of Catholic tradition.

The reporter then pressed the Pope, asking: "But forever, forever? Never, never?"

Francis responded: "If we read carefully the declaration by St John Paul II, it is going in that direction."

Francis has previously said the door to women's ordination is closed, but proponents of a female priesthood are hoping that a future pope might overturn the decision, particularly because of the shortage of priests around the world.

The Catholic Church teaches that women cannot be ordained priests because Jesus willingly chose only men as his apostles. Those calling for women priests say he was only following the norms of his time.

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In August, Francis set up a commission to study the role of women deacons in early Christianity, raising hopes among equality campaigners that women could one day have a greater say in the 1.2 billion-member Church.

Deacons, like priests, are ordained ministers and must be men. They cannot celebrate Mass, the Catholic Church's central rite, but they are allowed to preach and teach in the name of the Church, and to baptise and conduct wake and funeral services.

The Church barred women from becoming deacons centuries ago.

Scholars debate the precise role of women deacons in the early Church. Some say they were ordained to minister only to other women, for instance in baptismal immersion rites. Others believe they were on a par with male deacons.