The British Humanist Association hasn't had the smoothest ride in its attempt to spread the atheist word through advertising.

In 2009, it was advised by the Committee of Advertising Practice to add the word "probably" into its bus campaign slogan declaring: "There's probably no God. So stop worrying and enjoy your life." According to the committee: "The inclusion of the word 'probably' makes it less likely to cause offence, and therefore be in breach of the advertising code."

Now, the pesky atheists are in trouble again -- they were told that their new campaign was too offensive to run on railway station billboards. So what crime against God and good taste have they committed this time?

Well, the association is trying to persuade non-believers to declare themselves as such on the census form, so it will be a more accurate representation of the religious/non-religious make-up of the country. It's an important point. The 2001 census's figure of 71.8 per cent of Britons being Christian is often invoked by policymakers in debates over, for example, faith schools. (And yes, that's the same census where 390,127 people said they were Jedis. When will their faith school needs be taken in account, eh?)

According to this blog post by the New Humanist, the association came up with the slogan: "If you're not religious, for God's sake say so." And that's what seems to have caused the trouble, with the companies which own the advertising space telling them the phrase "for God's sake" could cause "widespread and serious offence". The amended tagline now reads, "Not religious? In this year's census, say so."

I asked Andrew Copson, chief executive of the BHA, who had made the decision to reject the adverts. He said the BHA was told by CBS Outdoors, who administer billboards in stations, that its "franchise partners" had rejected them. (I called CBS Outdoors to check this, but their press office haven't got back to me yet.)

As Paul Sims points out on the New Humanist's blog, it seems a very odd decision. Pro-religious adverts, such as those for the Alpha course (a Christian programme), the Christian Party and the Trinitarian Bible Society, have recently appeared on public transport. "The Trinitarian adverts said 'the fool hath said in his heart, there is no God'," Copson told me. "That seems more offensive, if you want to look at it that way! It's ridiculous."