LONDON-- More than 5,000 Catholics attend Masses every Sunday at Our Lady Mother of the Church, packing the old stone church so completely that loudspeakers were recently installed outside so the overflow crowd can hear.

"I go to Mass here every Sunday and sometimes during the week, too," said Dominika Marszalkowska, 27, an engineer from Poland who was among the throng listening to the 11:30 am. Mass last Sunday in the chilly February air.

Marszalkowska, who said she earns more managing a London restaurant than as an architectural engineer in her native country, is one of hundreds of thousands of new immigrants transforming the Catholic Church in England. The surge in devout newcomers is so strong that many analysts said Catholics now surpass Anglicans in weekly attendance.

Since King Henry VIII renounced his Catholicism in the 16th century, Anglicans and Catholics have often had a difficult relationship. Queen Elizabeth II is the official head of the Church of England, and it is still a rule that any member of the royal family who marries a Catholic forfeits their place in the line of succession.

There are still far more people here who consider themselves Anglican. But for millions of English, the only connection with the church of their baptism is attendance at an occasional funeral, wedding or holiday service -- leading some to joke that the "C" and "E" in the Church of England's abbreviation stand for "Christmas" and "Easter." With so few churchgoers, some old and ornate Anglican churches are being turned into apartments, gyms and cafes.

In the past few years, roughly the same number of Catholics and Anglicans have been attending Sunday services in any given week -- about a million each, according to spokesmen for both churches. But now, "ethnic congregations are exploding," said Francis Davis, author of a new report by the Von Hugel Institute at Cambridge University on the phenomenal influx of Catholic immigrants.

Davis said that as many as 500,000 Catholic immigrants, many of them very devout, are causing Catholic church attendance "to take off." One London church was down to 20 members when it introduced Masses in Portuguese, and suddenly about 1,400 people were attending Sunday Mass, Davis said.

Arun Kataria, a spokesman for the Church of England, said that weekly participation at services is only one way to measure the strength of a church. He said that 26 million people in England are baptized Anglicans, compared with 4.2 million baptized Catholics, and that the number of Anglican worshipers is holding steady. Of the many immigrants coming to Britain, he said, most tend to live in cities and have not affected the religious makeup of the countryside.

Kataria said that although "clearly a great many immigrants are coming in," not all of them are flocking to Catholic churches.

But many are.

"The face of London is changing, and with it, the church," said Cardinal Cormac Murphy-O'Connor, the leader of the Catholic Church in England and Wales, when he addressed the topic recently. He said immigrants were filling 90 percent of low-paid jobs, working as cleaners, builders and caterers, and he estimated that they make up almost a third of the city's workforce. "A very high proportion -- notably from Central and Eastern Europe -- are practicing Catholics," he said.

Murphy-O'Connor said it was a challenge for the church to fulfill the needs of immigrants, some of whom end up homeless and exploited. Last May, he said from the pulpit that he backed a government amnesty for long-term illegal residents, prompting an estimated 2,000 immigrants in the cathedral to burst into applause.

Migration is also swelling the ranks of Catholics in Northern Ireland, where the Catholic minority has long been feuding with the Protestant majority. Three decades of armed conflict, largely pitting Catholics against Protestants, cost more than 3,600 lives before a cease-fire was negotiated. Projections now show that immigration, along with higher birthrates among Catholics, may soon leave the population of Northern Ireland evenly divided between the two faiths.

The religious makeup of the province's police force has been a major hurdle in cementing peace. Northern Irish Catholics for decades have mistrusted and boycotted the police, a Protestant-majority force that Catholics viewed as biased against them. Officials have been trying to recruit more Catholics, and last month they got an unexpected boost when 1,000 Poles signed up -- nearly all of them Catholic immigrants.

The influx of new immigrants is generally traced to 2004, when the European Union expanded from 15 countries to 25. That meant workers from the new member countries -- eight of them in Eastern Europe -- were legally allowed to work in the United Kingdom. Poland, which is more than 90 percent Catholic, has by far the largest population of the new E.U. countries.

Official British government statistics show that about 490,000 migrants, 300,000 of them Poles, have arrived since 2004. Polish authorities estimate that the number of Polish workers here is far higher, about double the official figure, at 600,000. Thousands of Polish migrants continue to arrive at London bus stations and airports every week.

"It is very, very good, but sometimes it can be difficult" to have so many parishioners, said Tadeusz Wyszomirski, a parish priest at Our Lady Mother of the Church in west London.

Even though he recently added a seventh Sunday Mass -- all of them are in Polish -- the large church with grand stained-glass windows still overflows at most services. Some people kneel in the aisles, others stand outside even in London's cold winter rain. Crowds also flock to the church's three daily Masses in Polish on weekdays.

"I hope it continues to grow," he said. But the five priests are very busy, he added, trying to keep up with all the weddings, baptisms and home visits to the sick.

At Sunday's 11:30 a.m. Mass, Marszalkowska stood outside, listening to prayers over loudspeakers with her 9-week-old baby and her father, who is visiting from Poland. She said she speaks both English and Polish but looks forward to hearing the Mass in her native language.

Afterward, she joined other churchgoers in the basement for tea and Polish pastries -- including huge slices of a very popular apple cake.

Monika Swierczyusko, who came here two years ago from Poland, was working behind the counter. She said she works in a factory six days a week and helps out at the church every Sunday from 7 a.m. to 9 p.m.

"I don't understand people who don't like to go to church," she said, as another thousand people settled in upstairs for the next Mass.