Norwich has the highest proportion of the population in England and Wales reporting no religion, at 42.5%, according to the census.
It is not the first time an area in the east of England has taken the title.
In 2009, a survey by think tank Theos suggested it was the most non-religious region of the UK, with almost half of those surveyed saying they believed the theory of evolution made God obsolete, and more than 80% disagreeing with creationism and intelligent design.
Andrew Copson, from the British Humanist Association, which ran a campaign to persuade people to tick the "no religion" box last year, says it's not easy to know exactly why Norwich stands out.
"We know that it tends to be more young people, whites and males that say they are non-religious," he says. "Norfolk also has a lot of free churches, and we know they tend to die out more quickly than other churches - so that might be a factor."
Norwich does have two cathedrals and is said to have more standing medieval churches - 32 - than any city north of the Alps.
Keith Morris, editor of Network Norfolk, which reports on the Norwich and Norfolk Christian community, says his experience contradicts the census findings.
"In the last year we have reported on a number of new church congregations, including those serving mainly Chinese and African residents in Norwich itself, " he says.
Other posts on the UK Census 2011;