Once, it was British missionary priests who went to Africa and Asia on missions to convert. Now, the thriving churches in the developing world are returning the favour, sending clergy back to Europe to help meet a growing shortage.

One reason for the declining number of Catholic priests in Europe is because selection is so much more rigorous, according to a Nigerian priest who works in the UK.

Father Victor Darlington, a parish priest in Camberwell in the Southwark Archdiocese, sees himself as a "missionary" as well as a parish priest. He also teaches at St John's seminary Wonersh, where he says there are lower numbers of trainee priests but the quality is higher.

"A lot of people still wish to become priests, but they may not have all the gifts the priesthood demands. There is a much more rigorous system of discernment now to meet changing times."

In Africa, where numbers of priests are still soaring, the same rigors will soon be applied to selection as are required here already, he told Christian Today.

Father Victor, who has a doctorate from the Angelicum in Rome, came to the UK to serve a sabbatical and ended up teaching at the seminary after he was talent spotted by a bishop. He's been parish priest at the Church of the Sacred Heart for four years.

He said he did not believe the number of Catholics was declining so much as the way they lived and expressed their faith. For example, there might be fewer in church but every year when he leads the Corpus Christi procession through the streets, which takes place again this coming Sunday, people emerge from restaurants and bars and make the sign of the cross as he passes.

"It is not true to say the faith is dying or in decline in Europe. A lot of believers due to circumstance do not come to church on Sunday. But they are still believers, they are still Christians."

From Uromi diocese in mid-west Nigeria near Benin city, he has stayed in the UK because he believes in a duty to serve where he feels called.

New research this week showed that since 1980, the number of Catholic parishes has increased by 7 per cent but the total population has grown faster. There are now more than 5,500 Catholics per parish, compared to 3,759 in 1980. In Asia and Africa, where Catholic population growth was fastest, the number of parishes doubled. In every other part of the world there was also a growth in the number of parishes except in Europe where there was a decline of 12 per cent, a loss of 16,669 parishes since 1980.

The research, by the Center for Applied Research in the Apostate at Georgetown University, Washington DC, showed there are now 1.2 billion Catholics in the world, up from 7.83 million in 1980. There has been a dramatic increase in the number of Catholics in Africa, up 238 per cent since 1980, compared to just six per cent growth in Europe.

But this growth has not been matched by a sufficient growth in the number of clergy. To maintain the current ratio of Catholics per parish in 2050, the number of parishes globally would need to increase by 75,000 to 300,000. The big problem facing the Church is the shortage of priests to serve in these parishes, the report says. Globally, by 2012 the Church had just 713 more priests than it did in 1980. The biggest decline was Europe, which has lost nearly a quarter of its priests, a loss of 56,830 in total since 1980. About a third of parishes in Europe have been without a resident priest since 1980.

In Africa, by comparison, the number of priests more than doubled, leading to a growing phenomenon where African and Asian priests are being used in the US and Europe. "It is likely that Europe faces a future of fewer priests and more parish closures while growth in priests and parishes is likely to continue in Asia and Africa," the report says.

"Perhaps the most challenging aspect for the Catholic Church in Europe is maintaining a sufficient number of clergy to staff its parishes and still service a sizeable and historic Catholic population. Europe may need increasingly to rely on international priests from Africa and Asia in the future."

The report adds that one of the reasons for the declining number of priests is becaue the Church consistently ordains too few to make up for those it loses to mortality and defection. Although the rate of loss has slowed, the Church in Europe needs to ordain 1,500 more priests a year just to keep numbers stable, an unlikely target given the low numbers undergoing formation in seminaries.

Bishop Patrick Lynch, an auxiliary in Southwark, told Christian Today: "The ratio of Catholics per priest in the UK is quite healthy – in the region of one priest to 1,000 parishioners. In some parts of the world, such as Latin America, the ratio could be one priest to 10,000 parishioners.

"The number of Catholics attending Mass from Africa, Asia and South America has increased dramatically over the last ten to fifteen years, particularly in urban and city areas. From diocese to diocese, much careful thought, consultation and prayer has gone into how best to respond to that pastoral reality.

"In the Archdiocese of Southwark, there are a number of diocesan priests, religious orders and missionary societies who after a practical programme of preparation serve our Catholic parishes and communities and the Archdiocese is flourishing with people and with life."