Pope Francis said on Tuesday he believes the Roman Catholic Church's ban on women becoming priests is forever and will never be changed, in some of his most definitive remarks on the issue.

He was speaking aboard a plane taking him back to Rome from Sweden, in the freewheeling news conference with reporters that has become a tradition of his return flights from trips abroad.

A Swedish female reporter noted that the head of the Lutheran Church who welcomed him in Sweden was a woman, and then asked if he thought the Catholic Church could allow women to be ordained as ministers in coming decades.

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

Francis 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 that 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.

In reaction, The Women's Ordination Conference (WOC), a group that promotes a female priesthood, said in a statement that "Patriarchy Will Not Have the Last Word".

WOC said it was "profoundly disappointed" by the pope's comments, calling the document the pope referred to "outdated, fallible and painful".

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.

"The Church cannot be afraid to examine customs when they no longer communicate or resonate with the Gospel. A Church that is not open to the gifts of half of its membership is unsustainable and out-of-touch with the needs of its people," WOC said.

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 baptize 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.