Christians could be in a minority in England and Wales by 2018, according to analysis of the latest census results, which have revealed a fall of more than four million in the number of people who describe themselves as adhering to the faith.
The decline in Christianity came amid a wider fall in the number of people professing belief in any religion, although that trend was partially offset by an increase of 1.6 million in the number of Muslims.
Other findings showed that residents of England and Wales were increasingly non-white and foreign-born.
Secularists said the trend was a warning to the church that their conservative attitudes were not playing well with the public.
The Office of National Statistics revealed the decline at the same time as the Church of England's house of bishops met to discuss the crisis over the synod's decision last month to reject the ordination of female bishops. It also came as the government responded to religious opposition to same-sex marriage by announcing proposed legislation explicitly stating that it will be illegal for the Church of England and the church in Wales to marry same-sex couples, and that canon law, which bans same-sex weddings, will continue to apply.
Christianity remains the largest religion in England and Wales, with 33.2 million people, or 59 per cent of the population, saying they follow the faith. But 14.1 million people, around a quarter of the population and an increase of 6.4 million over the decade, said they had no faith at all.
A spokeswoman for the Church of England said the figures "confirm we remain a faithful nation" but admitted "the fall in those choosing to identify themselves as Christians is a challenge".
"One of the reasons may well be fewer people identifying as 'cultural Christians', ie, those who have no active involvement with churches and who may previously have identified as Christian for cultural or historical reasons," said Reverend Arun Arora, director of communications for the Archbishop's Council.
Ahead of the 2011 census the British Humanist Association used the slogan: "If you're not religious, for God's sake say so!", to encourage the non-religious to tick the "no religion" box. The BHA has now calculated that if the trend continues at a linear rate, Christians would be in the minority by September 2018.
"This is a really significant cultural shift," said the BHA chief executive, Andrew Copson. "To see such an increase in the non-religious and such a decrease in those reporting themselves as Christian is astounding."
Only 13,832 people explicitly identified themselves as humanists, far fewer than the 176,632 claiming to be Jedi Knights.
Muslims in 2011 numbered 2.7 million, up from 1.55 million in 2001. Muslims now make up 4.8 per cent of the population compared with 3 per cent in 2001.
The overall population of England and Wales was 56.1 million, an increase of 3.7 million or 7 per cent since 2001. Foreign-born residents rose by 63 per cent from 4.6 million in 2001 to 7.5 million.
White Britons now make up 80 per cent of the population, at 45.1 million people, down from 87 per cent in 2001. Some 2.5 percent are ethnic Indian and 2 per cent ethnic Pakistani.
Additional reporting by Agence France-Presse