Scientist and atheist Richard Dawkins backed the campaign
An atheist campaign claiming "There's probably no God" has been reported to the advertising regulator.
Posters with the slogan appear on 800 buses in England, Scotland and Wales, as well as on the London Underground.
But organisation Christian Voice has complained to the Advertising Standards Authority saying they break rules on substantiation and truthfulness.
The British Humanist Association, which backed the campaign, said it was not taking the complaint seriously.
The ASA's code states "marketers must hold documentary evidence to prove all claims". The regulator said it would assess the complaint and decide whether to contact the advertiser.
'Peals of laughter'
The adverts contain the slogan: "There's probably no God. Now stop worrying and enjoy your life."
But Stephen Green, national director of Christian Voice, said: "There is plenty of evidence for God, from people's 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."
The campaign was dreamed up by comedy writer Ariane Sherine and was supported by scientist and vocal atheist Richard Dawkins.
Hanne Stinson, chief executive of the British Humanist Association, 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."