The boom surpasses the rise in sales of books in categories such as history, which have grown by 38 per cent, and politics, up by 30 per cent, confirming that religion has become a pivotal topic in the early 21st century.

But the statistics may not make uplifting reading for believers. The most popular 'religious' book, says Amazon, is The God Delusion, an anti-faith polemic by Richard Dawkins, the academic who has been dubbed 'Darwin's rottweiler'. Second is God is Not Great: How Religion Poisons Everything, another broadside at holy citadels, by the journalist Christopher Hitchens.

Amazon said that the third most popular book in the category was Jesus of Nazareth by the Pope, followed by a perennial favourite among readers seeking spiritual fulfilment, Paulo Coelho's The Alchemist: A Fable about Following your Dream and a riposte to Dawkins entitled The Dawkins Delusion? Atheist Fundamentalism and the Denial of the Divine by Alister and Joanna McGrath.

But it is The God Delusion that has driven the growth of the category. Between April and June it was the fourth-bestselling book of all, beaten only by the two editions (children and adult) of Harry Potter and the Deathly Hallows and celebrity chef Gordon Ramsay's latest offering. However, the publication of The God Delusion last year also prompted a 120 per cent increase in sales of the Bible.

Amy Worth, books manager at Amazon, said: 'The God Delusion has been one of the bestselling books of the past year. People are interested in the debate it has sparked. There are 524 readers' comments on our site. The comments are both pro and against and it's clear that religious people are buying it. The Dawkins Delusion has also been successful, although when we had a customer-vote on The Dawkins Delusion versus The God Delusion, the winner was The God Delusion.'

Other books challenging religion have included Daniel Dennett's Breaking the Spell and AC Grayling's Against All Gods. Grayling, professor of philosophy at Birkbeck College, University of London, said they had found an audience following the terrorist attacks of 11 September, 2001. 'I think 9/11 has changed the nature of the debate tremendously,' he said. 'A decade ago people wouldn't say "I'm a Christian" at a dinner party. You would no more speak about your religious belief than you would your sex life.

'But after 9/11 we no longer think people should be treated differently or given exemption from certain laws because they believe something. Secularists are now saying, "OK, believe in what you like, believe in fairies at the bottom of the garden if you want to, but don't force your beliefs on us or our children, and don't expect preferential treatment." To allow religious organisations more privileges and influence than a political party or trade union, for example, is to distort public debate. People are waking up to the fact it is anomalous.'