The British Humanist Association’s adverts carry the slogan “There’s probably no God. Now stop worrying and enjoy your life.”, and will appear on 600 buses across the country, including 30 of London's bendy buses, as well as London tube stations.
The posters are the atheist answer to a number of high profile Christian advertising campaigns on London buses and billboards, notably the Alpha Course, whose posters ask, "Is this it?" and "If God did exist, what would you ask him?"
They have been publicly endorsed and partially financed by prominent atheist Professor Richard Dawkins, who told the BBC that the ad campaign was designed to make people think, an action he said was “anathema to religion”.
The public theology think tank Theos has welcomed the campaign, saying it will encourage more people to think about the existence of God.
"We think that the campaign is a great way to get people thinking about God. The posters will encourage people to consider the most important question we will ever face in our lives,” said Theos Director Paul Woolley.
"The slogan itself is a great discussion starter. Telling someone "there's probably no God" is a bit like telling them that they've probably remembered to lock their front door. It creates the doubt that they might not have done so.”
Mr Woolley said that a new Theos research study due to be published next month had revealed that there were as many people finding God in Britain today as there were losing people losing their faith.
“So this campaign is speaking into a very live debate,” he said.
Mike Elms, a fellow of The Marketing Society and former chief executive of ad agencies Ogilvy & Mather and Tempus/CIA, said that the campaign could play a role in the revival of Christianity.
"For too long, the British public has been able to dodge the 'God choice' - is there or isn't there? - by scribbling ‘C of E’ on their hospital admission form,” he said.
“But now atheists are challenging us to make that choice one way or another. The atheist campaign opens the door toward a very public debate on the existence and nature of God."
The head of Church Army, Mark Russell, has also previously expressed his support for the adverts in his blog, ‘Mark Russell’s reflections’.
"I love that the advert says 'probably'," he wrote, "so it seems the atheists are not sure if there is a God or not!! I hope people will take time out from the busyness of their everyday lives and think openly about the issues."