Initial results suggested only a 10% decline, but the ONS found that figure had been boosted by 1.2 million foreign-born Christians.
The new figures also show most Christians are over 60, and for the first time less than half of young people describe themselves as Christian. As a result, the ONS calculates that in a decade only a minority will classify themselves as Christians.
Christianity is still the main religion in Britain, with over half of the population identifying as believers. But this could change as the British Muslim population has surged dramatically over the past 15 years, increasing by 75% in England and Wales. The 2011 census puts the Muslim population in Britain at around 5%.
Dr Fraser Watts, a Cambridge theologian, said it was ‘entirely possible’ people identifying as Christians could become a minority within the next decade on the basis of the figures. ‘It is a worrying trend and confirms what anyone can observe ? that in many churches the majority of the congregation are over 60,’ he said.