Pope Francis has said he believes the Roman Catholic Church's ban on women becoming priests will be in place forever, 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.

Pope Francis believes women will never be permitted to be ordained by the Catholic Church

'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.

A 1994 document by Pope John Paul II closed the door to women being allowed to become Catholic priests

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.

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.