Roman Catholicism is set to become the dominant religion in Britain for the first time since the Reformation because of massive migration from Catholic countries across the world.

Catholic parishes will swell by hundreds of thousands over the next few years after managing years of decline, according to a new report, as both legal and illegal migrants enter the country.

It says that the influx of migrants could be the Catholic community’s “greatest threat” or its “greatest opportunity”.

While in some places the Catholic Church has responded positively, in others it has been “overwhelmed” by the scale of the challenge. The growth of Catholicism in Britain comes as the established Church of England and the Anglican provinces in Scotland, Wales and Ireland face continuing, if slow, decline.

Average Sunday attendance of both churches stood even at nearly one million in 2005, according to the latest statistics available for England and Wales, but the attendance at Mass is expected to soar.

A Church of England spokesman said: “I don’t think you can talk in terms of decline in the Church of England. It is fairly clear that with small fluctuations the worshipping population of the Church of England is 1.7 million a month. That is actually a stable figure.”

The report describes how many migrants have few or no documents, little or no English, no job to go to and nowhere to live.

The Catholic Church is the first port of call for thousands when they find themselves in difficulty, with up to 95 per cent from countries such as Poland being practising Catholics. Some churches find that they are being used as both job centres and social welfare offices. Most of the migrants settle in London, where some parishes are putting on Sunday Masses from 8am to 8pm to cope, the report, carried out by the Von Hugel Institute at Cambridge, found.

The report calls on the Catholic hierarchy to act urgently to help the migrants and their hard-pressed clergy by investing thousands of pounds in new resources.

Officially the Church is welcoming the migrants, but nearly all bishops and clergy have been taken by surprise by the influx, which took off last year and has yet to be reflected in official Mass attendance and membership figures.

But they acknowledge that the immigration is changing the face of Catholicism across Britain.

From being an Irish-English church in a mindset of managing steady decline, the Church has within the space of 12 months found itself having to countenance an unprecedented expansion and change in its ethnic make-up.

Figures for 2005 show that there are 4.2 million Catholics in England and Wales, under one fifth the 25 million baptised Anglicans and double the number of Muslims.

But the real Mass attendance figure is higher by many hundreds of thousands. Precise numbers are impossible to obtain because of the irregular status of so many of the migrants, who prefer to keep a low profile. Some would only talk to researchers for the report through their priests, and some clergy even refusedto be interviewed for fear of attracting attention.

But the head of the Polish vicariate told The Times that the number of Poles in London had doubled since their country’s EU accession to at least 600,000. According to the report, the number recorded attending Mass represents a fraction of the total number of baptised Catholic migrants now in London.

The Catholic dioceses of Brentwood, Southwark and Westminster, which cover Essex, London and Kent, commissioned the report to investigate the needs of migrants in London after a Mass in Westminster last May gave an indication of the scale of the change.

Researchers at Cambridge surveyed 1,000 migrants from diocesan parishes, ethnic chaplaincies and the Polish vicariate, ran focus groups and interviewed clergy.