The posters, which encourage people to tick the 'no religion' box if they do not believe in God, were judged too likely to offend

Three posters appealing to people who are not religious to declare themselves as such in this year's census have been banned from appearing in railway stations.

The posters, bearing the slogan "If you're not religious, for God's sake say so", have been refused by the companies that own the advertising space, which say they are likely to cause offence.

The British Humanist Association (BHA), which published the posters, said it was astonished that such an everyday phrase should be deemed too contentious for public display. "It is a little tongue-in-cheek," said the BHA chief executive, Andrew Copson, "but in the same way that saying 'bless you' has no religious implication for many, 'for God's sake' is used to express urgency and not to invoke a deity.

"This censorship of a legitimate advert is frustrating and ridiculous: the blasphemy laws in England have been abolished but we are seeing the same principle being enforced nonetheless."

The posters ask those who are not religious to tick the "no religion" box when they fill in forms for the 2011 census.

"We used to tick 'Christian' but we're not really religious. We'll tick 'No Religion' this time. We're sick of hearing politicians say this is a religious country and giving millions to religious organisations and the pope's state visit. Money like that should go where it is needed," says one of the banned posters.

The ban followed advice from the Advertising Standards Authority's committee of advertising practice that the advert had the potential to cause widespread and serious offence.

The poster display company involved also said it did not want to take adverts relating to religion.

The British Humanist Association has amended the campaign slogan on the adverts to read simply: "Not religious? In this year's census say so." The posters are being displayed from this weekend on 200 buses in London, Manchester, Leeds, Newcastle, Birmingham, Cardiff and Exeter.

The Humanist Association says that those who profess to have no religion rose from 31% in 1983 to 51% in 2009 and believes that many who ticked "Christian" in the 2001 census did so for cultural rather than religious reasons.

The organisation argues that unless this year's census gives a more accurate picture of the non-religious population, the data will be used to justify increased state-funding for faith schools and other religious organisations.

The Advertising Standards Authority says that in 2009 the Christian party's "There definitely is a God. So join the Christian Party and enjoy your life" became the most complained-about non-broadcast ad ever, attracting 1,204 complaints. The ad was a riposte to the British Humanist Association's "There is probably no god, now stop worrying and enjoy your life", which attracted 391 complaints.