A new analysis of the 2011 census shows that a decade of mass immigration helped mask the scale of decline in Christian affiliation among the British-born population – while driving a dramatic increase in Islam, particularly among the young.
It suggests that only a minority of people will describe themselves as Christians within the next decade, for first time.
Meanwhile almost one in 10 under 25s in Britain is now a Muslim.
The proportion of young people who describe themselves as even nominal Christians has dropped below half for the first time.
Initial results from the 2011 census published last year showed that the total number of people in England and Wales who described themselves as Christian fell by 4.1 million – a decline of 10 per cent.
But new analysis from the Office for National Statistics shows that that figure was bolstered by 1.2 million foreign-born Christians, including Polish Catholics and evangelicals from countries such as Nigeria.
They disclosed that there were in fact 5.3 million fewer British-born people describing themselves as Christians, a decline of 15 per cent in just a decade.
At the same time the number of Muslims in England and Wales surged by 75 per cent – boosted by almost 600,000 more foreign born followers of the Islamic faith.
While almost half of British Muslims are under the age of 25, almost a quarter of Christians are over 65.
The average age of a British Muslim is just 25, not far off half that of a British Christian.
Younger people also drove a shift away from religion altogether, with 6.4 million more people describing themselves as having no faith than 10 years earlier.
Secular campaigners said the new figures showed that Christianity had now dropped below “critical mass” making the case for disestablishing the Church of England stronger.
But the Church insisted that while there had been a significant drop in “nominal” Christians, the core of the Church remained firm.
Prof David Coleman, Professor of demography at Oxford University, said: “This is a very substantial change – it is difficult to see whether any other change in the census could have been remotely as big.
“But I wonder how far it reflects an overarching change in society where it is more acceptable more normal to say that you are not religious or are not Christian.”
Dr Fraser Watts, a Cambridge theologian, said it was “entirely possible” the people identifying themselves as Christians could become a minority within the next decade on the basis of the figures.
“It is still pretty striking and 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.
Keith Porteous Wood, executive director of the National Secular Society, said the long-term reduction of Christianity, particularly among young people, was now “unstoppable”.
“In another 20 years there are going to be more active Muslims than there are churchgoers,” he said.
“The time has now come that institutional Christianity is no longer justified, the number has dropped below critical mass for which there is no longer any justification for the established Church, for example, or the monarch going through a religious ceremony at coronation.
“The expressions of optimism by the church are just completely misplaced.”
But a spokesman for the Church of England said: “These figures highlight the diversity of Christianity in this country today, something which has been increasing for decades and shows the relevance of Christianity to people from all backgrounds.
“These figures once again confirm that this remains a faithful nation and that the fall in the numbers identifying themselves as Christians is a challenge but – as you can see from the stability of Church of England attendance figures – the committed worshipping centre of the church remains firm.
“The challenge to the Church is to reconnect with the nominal.”