The British Humanist Association has dismissed claims from a fundamentalist Christian preacher that a new ad campaign breaks industry codes of practice.

It is the UK’s first ever atheist advertising campaign.

Stephen Green, leader of Christian Voice, is a high-profile opponent of gay rights and has protested at Pride events.

The new adverts appear on 800 buses across Britain.

“There is probably no God,” they read.  “Now stop worrying and enjoy your life.”

The £140,000 campaign, which will also use ad space on the London Underground and two large LCD screens on Oxford Street, will run for four weeks and has been funded by leading humanists such as scientist Richard Dawkins.

Mr Green has complained to the Advertising Standards Authority, claiming that the rules state any ad must be factually correct.

“According to one national newspaper, some atheist supporters of the campaign were disappointed that the wording of the adverts did not declare categorically that God does not exist, although there were fears that this could break advertising guidelines,” he said.

“I believe the ad breaks the Advertising Code anyway, unless the advertisers hold evidence that God probably does not exist.

“The advertisers cannot hide behind the ASA’s ‘matters of opinion’ exclusion, because no person or body is named as the author of the statement. It is given as a statement of fact and that means it must be capable of substantiation if it is not to break the rules.

“There is plenty of evidence for God, from peoples’ personal experience, to the complexity, interdependence, beauty and design of the natural world.

“But there is scant evidence on the other side, so I think the advertisers are really going to struggle to show their claim is not an exaggeration or inaccurate, as the ASA code puts it.”

Mr Green claimed that the Bible would be immune from any questions of substantiation.

“The Bible says ‘the wages of sin is death but the gift of God is eternal life through Jesus Christ our Lord'” is entirely factual.

“The Bible does say that.

“The statement “Jesus said, ‘I am the way, the truth and the life – no-one comes to the Father but by me,'” to take another example, is a Biblical quote, from the same Bible which is part of our Christian Constitution and upon which witnesses promise to tell the truth in court.”

British Humanist Association chief executive Hanne Stinson said:

“I’ve sought advice from some of our key people here, but I’m afraid all I’ve got out of them so far is peals of laughter.

“I am sure that Stephen Green really does think there is a great deal of evidence for a God (though presumably only the one that he believes in) but I pity the ASA if they are going to be expected to rule on the probability of God’s existence.

“However, if they do investigate we will be very happy to respond.”

The ASA said they are considering whether to take any action.

The campaign is a response to a series of evangelical Christian adverts running on buses in June 2008, which featured the URL of a website saying all non-Christians were going to hell.

Comedy writer Ariane Sherine suggested the rational, positive slogan to reassure people who may have been scared by the evangelical adverts.

Christian Voice is itself under investigation by the ASA for an advertorial in the New Statesman that contained a prediction about teenage infertility that could not be substantiated.

Mr Green is one of Britain’s most outspoken homophobes.

In November bookseller Waterstones chose to cancel an event at a Cardiff store featuring a gay Welsh poet when Christian Voice threatened a protest.

Patrick Jones was due to read from his new collection of poems, Darkness is Where the Stars Are, and sign copies.

Some of the poems explore issues of faith, though Christianity is not singled out.

Christian Voice claim the poems are blasphemous.

When it emerged that Mr Green was to protest, the shop decided to cancel the event.

In 2006 he was cleared of public order charges after handing out anti-gay leaflets at Cardiff Mardi Gras.

In 2007 he protested at both London and Brighton Pride.

Mr Green questioned the presence of racist group the National Front at Pride events, on the basis that “homosexuals are not black or Asian as a rule.”

The controversial preacher made his observation as part of his report on the group’s website about his Brighton protest, where he had to be given police protection as he and a handful of his followers displayed homophobic posters at the entrance to Preston Park.