The figures on religion for the 2011 census show that of nearly 249,000 people in Derby, 52.7% classed themselves as Christian, compared to 66.4% in 2001.
And there was a higher portion recording they had no religion, 27.6%, compared to 15.9% 10 years ago.
Derby’s figure for Christianity was lower than others in the region. In Erewash, 57.8% said they were Christian with 33.9% saying they had no religion.
Elsewhere the numbers topped 60% for Christianity, with the highest figure being in Derbyshire Dales – 68.7%.
The question on religious beliefs was only introduced in the 2001 census.
Professor Paul Weller, an expert in Inter-Religious Relations at the University of Derby, said that the latest figures are similar to those from other social surveys where the question normally posed is whether people believe in a god, spirit or other being.
“People will often respond no to those sorts of questions but still say they are Christian because they give their religion more as a cultural answer,” he said.
“What seems to have happened is the figure that’s now coming through the census is more in line with the figures on those social identity surveys and a greater proportion now identify as non-religious compared to 10 years ago because they are now understanding the question.”
The number of people identifying themselves as “Muslim” has increased in the city from 4.5% ten years ago to 7.6%.
Professor Weller said: “I think it was always thought, particularly in Derby, that we would likely see an increase in the Muslim population through refugees in the early part of the decade.
“It is also the case that social geography means, on the whole, the Muslim population is of a younger age range than the Christian or Jewish. That means the reproduction at present is leading to a larger population growth.”
Apart from Christianity, other religions saw an increase, including Buddhism. In Derby, that has risen from 0.2% to 0.3%.
The 2011 Census also revealed that people describing themselves as something other than “white, British” increased between 2001 to 2011 by 20% in the East Midlands.
In Derby, the figure for those classing themselves as “white, British” was 75.3% compared to 95.8% in the rest of the county.
The figure for those classing themselves as “white, other” was 3.9% in Derby compared to just 1.2% in other parts of Derbyshire.