From next week, the BHA will be running posters on buses and trains telling the non-religious: “If you’re not religious then for God’s sake say so.”

The group says the data on religion in the 2001 census gave a “wholly misleading picture” of the religiosity of the UK.

The 2001 census was the first to ask British people ‘What is your religion?’, although answering was voluntary. Seventy-two per cent of the population in England answered the question by identifying themselves as Christians.

This year, the census in England and Wales will ask: ‘What is your religion?’, while in Scotland it will say: “What religion, religious denomination or body do you belong to?’

The BHA is critical of the question, arguing that it encourages people to identify themselves with a religion even though they may not be practising believers, thereby distorting the numbers who are genuine followers of a particular faith.

In a bid to address concerns over accuracy, the Office for National Statistics has introduced a small change to the tick options, replacing ‘none’ with ‘no religion’.

A spokesman for ONS defended the question to the BBC: “The religion question measures the number of people who self-identify an affiliation with a religion, irrespective of the extent of their religious belief or practice.”

The census takes place this year on March 27.