Fifteen percent of people in the UK say they have no religion, according to government figures.
Muslims were the second largest religious group with 1.6 million.
Focus on Religion, published by the National Statistics Office, draws on data from the 2001 Census – the first to include a religion question.
The results show that the Muslim population is the youngest with 34% under 16 in 2001.
It also shows that almost a third of UK Muslims of working age had no qualifications – the highest proportion for any religious group – and that they have the highest unemployment rate among men with 14%
The data shows that the Jewish population was the most heavily concentrated in London with 52% of Britain’s Jewish people living in the capital.
This compares with a quarter of Sikhs.
Less than four in ten – 37% – of Hindus living in the UK were born elsewhere with a similar proportion being born in Asia.
Young people with no religion were the most likely to be cohabiting with 16% of 16 to 24-year-olds doing just that.
Jews were the most likely to be self-employed at one in three, compared with one in five for Muslims and one in 10 for all other groups.
The data shows that the unemployment rate was higher for Catholics – 9% – than for Protestants – 5%.
An Office for National Statistics spokesman told BBC News Online: “Focus on Religion provides an important perspective to the question of ethnicity because some people like to be identified by their religion as well as their identity.
“There are statistics other than those gained from the 2001 Census which come from more recent sources so it brings it all together in a detailed profile of all the different faith groups.”