In one of the most holy weeks in the Christian calendar, a report says that in just over a generation the number of people attending Church of England Sunday services will fall to less than a tenth of what they are now.

Christian Research, the statistical arm of the Bible Society, claimed that by 2050 Sunday attendance will fall below 88,000, compared with just under a million now.

The controversial forecast, based on a "snapshot" census of church attendances, has been seized upon by secular groups as proof that the established church is in decline. But the Church of England has rejected the figures, saying they were incomplete and ignored new ways of worshipping outside the church network.

According to Dr Peter Brierley, former executive director of Christian Research, by 2030 just under 419,000 people will attend an Anglican Sunday service. By 2040 the number will be down to 217,200, falling to 153,800 five years later. By 2050, if the trend prediction is correct, only 87,800 will be attending.

The figures stand in contrast to the picture of faith described by the prime minister earlier this month. In a preface to a new report, Faith in the Nation, Gordon Brown said: "Faith in Britain today is very much alive and well. At the last census, more than three-quarters of the population said they belonged to a faith ... people's religious identities go right to the heart of their sense of themselves and their place in society and the world."

Keith Porteous-Wood of the National Secular Society said: "Church attendance has already been in decline for over 60 years, all over Britain, in all major denominations and across all age groups, except the over-65s. Independent statisticians now have enough data to predict confidently that the decline will continue until Christianity becomes a minority sect of largely elderly people, in little more than a generation."

The forecast was made by Christian Research in its annual statistical publication, Religious Trends. Benita Hewitt, the organisation's new executive director, said she accepted that the figures were disputed and stressed she did not believe they showed people were turning away from religion. "As with all forecasting, we are living in rapidly changing times at the moment and it is very difficult to predict what things will look like in the coming years," she said.

The Reverend Lynda Barley, head of research and statistics for the Archbishops' Council, said the figures represented only a "partial picture" of religious trends, adding: "Church life has significantly diversified so these traditional statistics are less and less meaningful in isolation."

Studies suggest figures for Sunday attendance represent only 58 per cent of the number of people who attend in an average month. Attendance at Church of England cathedral services has been growing , while church groups have attracted new congregations by holding meetings in venues such as pubs or at car boot sales.