The new analysis of the 2011 data released Friday shows Christianity has been losing ground even faster than thought among those born in the country, The Daily Telegraph reported.
A report last year said the Census found 4.1 million Christians, down 10 percent in a decade. That figure turns out to include 1.2 million recent immigrants, including Polish Catholics and evangelical Protestants from Nigeria and other African countries, with a 15 percent drop in observant Christians among those born in Britain.
The differences are starkest among those under 25 with almost one in 10 describing themselves as Muslim.
David Coleman, an Oxford University demographer, said the change is "substantial."
"But I wonder how far it reflects an overarching change in society where it is more acceptable, more normal to say that you are not religious or are not Christian," he said.