The Christian population of England and Wales has fallen by four million to 33.2 million in the past decade, the 2011 census reveals.
It comes as the Archbishop of Canterbury, Rowan Williams, says English cathedral congregations have grown in recent years.
The number of people describing themselves as having no religion rose from 15% to 25% of the population.
The British Humanist Association said it was a "significant cultural shift."
Nick Spencer, research director at think-tank Theos, said:
"These figures show that we have a plural religious landscape but that doesn't mean we're atheists. Digging deeper, we see that even those who say they have no religion often have a variety of spiritual beliefs but they don't want to associate these to religious institutions."
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