The number of people describing themselves as Christians has dropped 10% in England and Wales over the last five years.
In 2005, 77% of the population said they were Christian, but according to the Citizenship Survey published this week, that figure fell to 70% in 2010.
The government-commissioned survey looked at the views of more than 10,300 adults.
In the same period, the number of people saying they have no religion rose by 6%, from 15% in 2005 to 21% in 2010.
Christians were also far less likely to practise their religion than those of other faiths. Those most likely to practise their religion were Muslims.
The report said: “While Christianity remained the most prevalent faith in England and Wales, between 2005 and 2010 there was a steady decrease in the proportion of people who identified themselves as Christian.
“Christian people were must less likely than all the other main religions to say that they practised their religion, while Muslim people were most likely to practise their religion.”
Despite the fall in people affiliating themselves with Christianity, those who do are more committed, with an increase in the number of Christians saying they go to church regularly, from 31 per cent in 2005 to 33 per cent in 2010.
Earlier in the month, the British Social Attitudes Report also predicted the “long-term decline” of religion in Britain.
Of the 3,000 adults surveyed, it found a fall in the number of people identifying themselves as Christian from 50% to 42% in the last three years.
As the older, more religious generations are replaced with younger generations opting not to raise their children with a religion, the survey predicted that liberal attitudes towards social issues would continue to gain ground.
There may be an “increased reluctance … for matters of faith to enter the social and public sphere at all”, it warned.
That conclusion reflects the concerns of many Christians who feel that the law is increasingly turning against them.
Four Christians penalised for their beliefs in the workplace are taking their cases to the European Court of Human Rights.
The British Government has been criticised by the former Archbishop of Canterbury, Lord Carey, for failing to support their cases.