The march of secularism means Britain may no longer be a Christian country in just 20 years, a report said yesterday.
If trends continue, the number of non-believers is set to overtake the number of Christians by 2030.
Christianity is losing more than half a million believers every year, while the count of atheists and agnostics is going up by almost 750,000 annually
Research by the House of Commons Library found that while Christianity has declined, other religions have seen sharp increases.
In the last six years, the number of Muslims has surged by 37 per cent to 2.6million; Hindus by 43 per cent and Buddhists by 74 per cent. But the number of Sikhs and Jewish believers fell slightly.
The researchers said the number of Christians had only held up to the extent it has because of high levels of immigration over the last decade.
The findings help explain the increasingly forceful warnings from people of faith about the state of religion in Britain.
Last week a group of MPs and peers – Christians in Parliament – claimed public policy was promoting ‘unacceptable’ discrimination against Christians.
Yesterday the group’s chairman, former Tory justice minister Gary Streeter, warned that believers were having their faith ‘steamrollered’ by a ‘secular and hostile state’.
He said many were facing restrictions on their ability to practise their religion in public.
Cabinet minister Baroness Warsi also recently attacked ‘militant secularism’ and said the Church should continue to have influence over government policy.
Earlier this week campaigners said Christianity was being sidelined after it emerged that a council-run crematorium in Bath was to remove a glass window engraved with a four-foot cross to make it more suitable for other religions.
Secularists argue that Christians should no longer have privileged access in Parliament when the number who believe in God is declining so sharply.
Researchers came to their conclusion after studying the Labour Force Survey, which is carried out every year by the Office for National Statistics.
It is the most authoritative survey because of its regularity and its large sample size of 50,000.
It found that in 2010 there were around 41.1million Christians in Britain – down 7.6 per cent over the past six years.
There were around 13.4million non-believers, up 49 per cent over the same period.
The study, Religion in Great Britain, concludes: ‘Between the fourth quarter of 2004 and the fourth quarter of 2010, the Christian population fell from 78.0 per cent of the population to 69.4 per cent, while the group of people with no religion grew from 15.7 per cent to 22.4 per cent.
‘If these populations continue to shrink and grow by the same number of people each year, the number of people with no religion will overtake the number of Christians in Great Britain in 20 years, on this measure of religious affiliation.’
The Labour Force Survey asks people what religion they belong to, ‘even if you are not currently practising’ – a form of question which tends actually to underestimate the number who say they are non-believers.
The survey also shows people are more likely to define themselves as Christian the older they are.
Almost 90 per cent over 70 say they are Christian, compared with just over 50 per cent of those aged 30 to 34.
The researchers say this could be because ‘as children grow into young adults and form a religious identity independent of their parents, an increasing proportion are coming to regard themselves as having no religion’.
They point out that the decline of Christianity would have been far deeper had there not been such high levels of migration.
While the number of Christians born in the UK declined by 4.1million, the number born abroad increased by around 730,000.
The study also looked at the British Social Attitudes survey, which routinely finds greater numbers of non-believers.
In 2011, it found that 50 per cent of respondents did not regard themselves as being of any religion, compared with just 44 per cent who said they belonged to some form of Christianity.
However, the Library said this was a much smaller sample size, making it less reliable as a guide.
Despite the rising numbers following Islam, there are still 16 times as many Christians as Muslims.