Britain is no longer a country of believers but rather has entered a post-Christian era, a former archbishop of Canterbury has said.

Lord Williams of Oystermouth, who stood down as leader of the Church of England in December 2012, said the time of habitual worship was over and that a further decline of widespread faith was likely.

His comments, in an interview with the Sunday Telegraph, came after the prime minister was criticised for saying the UK should be "more confident about our status as a Christian country" and more evangelical about faith.

David Cameron's comments prompted fury from secular and atheist groups and led to the Liberal Democrat leader, Nick Clegg, an atheist, calling for the separation of church and state in England.

A poll for the newspaper also found that while more than half the public regard Britain as a Christian country, the majority of practising Christians are afraid to express their beliefs.

Williams, who is now master of Magdalene College, Cambridge, said that while Britain's "cultural memory is still quite strongly Christian", it was post-Christian in that habitual practice across most of the population was not taken for granted.

"A Christian nation can sound like a nation of committed believers, and we are not that. Equally, we are not a nation of dedicated secularists. I think we're a lot less secular than the most optimistic members of the British Humanist Association would think," he said.

Underlining Britain's changing relationship with religion, Williams added: "A Christian country as a nation of believers? No. A Christian country in the sense of still being very much saturated by this vision of the world and shaped by it? Yes."

Asked whether Britain would lose its faith altogether, he said: "Given that we have a younger generation now who know less about this legacy than people under 45, there may be a further shrinkage of awareness and commitment."

Williams added: "The other side is that people then rediscover Christianity with a certain freshness, because it's not 'the boring old stuff that we learnt at school and have come to despise'. I see signs of that, talking to youngsters here at Magdalene and in school visits. There is a curiosity about Christianity."

The Sunday Telegraph's poll, which surveyed 2,000 people online, found that 56% of people regard Britain as Christian, rising to 60% among men and 73% among over-65s.

Sixty-two per cent said the rise of religious fundamentalism had made Christians afraid to express their faith while 41% said they were not religious.