New figures from the 2011 Census show that the number of people who identify as Christians in England and Wales has fallen by 4 million in the last 10 years.
The data show numbers fell from 37.3 million in 2001 to 33 million last year.
The statistics came as the outgoing archbishop of Canterbury, Rowan Williams, claimed that English cathedral congregations are growing dramatically, challenging the claim made by secularists that the Church of England is fading in Britain.
Figures from the 2011 Census show the number of people declaring themselves to be atheists rose by more than 6 million, to 14.1 million.
"It should serve as a warning to the churches that their increasingly conservative attitudes are not playing well with the public at large," said Terry Sanderson, president of the National Secular Society. "It also calls into question the continued establishment of the Church of England, whose claims to speak for the whole nation are now very hard to take seriously."
Other polls have detected similar shifts. The 2012 British Social Attitudes Survey showed that only about half of Britons claim a religious affiliation, down sharply from 20 years ago when two out of three Britons did. Barely a quarter of young people identify themselves as religious.
The new figures show that Islam is the U.K.'s second-largest religion, at 2.7 million. Hinduism is third, at 817,000. The number of self-identified Jews rose by 3,000, from 260,000 to 263,000.