A think tank set up with the support of the heads of the Church of England and the Roman Catholic Church in England and Wales says an atheist bus advert campaign will "encourage people to think about God".

The British Humanist Association's adverts have made headlines over their provocative slogan, "There's probably no God. Now stop worrying and enjoy your life."

More than £30,000 has been raised for the four-week campaign, which will see the posters splashed across 30 of London's bendy buses next January.

Prominent atheist and author of "The God Delusion", Professor Richard Dawkins had offered to match donations up to £5,500.

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?"

Ariane Sherine, a comedy writer, prompted the "no God" advert campaign when she called for a "reassuring counter-advert" to the religious posters on public transport that she said "threaten passengers with eternal damnation", according to The Telegraph.

"This is absolutely brilliant and I'd like to thank everyone who donated for their support. The sky's the limit for atheists even if we don't believe there's anyone up there," she was quoted by the newspaper as saying.

More than five times the target of £5,500 has been gathered since the launch of a donation page on the Justgiving website on Tuesday.

The BHA's Hanne Stinson said that if the donations kept up, the organisation would consider rolling the adverts beyond London buses, to trains on the London Underground, and possibly transport networks in more cities across the UK.

Faith-based think tank Theos said it had donated £50 to the campaign out of a conviction that the campaign would prove to be "counter-productive".

"It tells people to 'stop worrying', which is hardly going to be a great comfort for those who are concerned about losing their jobs or homes in the recession," said Theos director Paul Woolley.

"And what does it tell us to do when we stop worrying? Volunteer overseas? Give money to charity? Campaign for the environment? No. It tells us to enjoy ourselves. It would be hard to come up with a more self-centred message than this."

He added, "Stunts like this demonstrate how militant atheists are often great adverts for Christianity."

The adverts have been well received by other Christians.

The Methodist Church's Spirituality and Discipleship Officer, the Rev Jenny Ellis, said: "This campaign will be a good thing if it gets people to engage with the deepest questions of life."

She also defended Christians against Dawkins' assertion that "thinking is anathema to religion", saying that Christians were called by Jesus to "love God with our minds as well as our hearts, souls and strength".

"Christianity is for people who aren't afraid to think about life and meaning," she added.

The head of Church Army, Mark Russell, expressed his support for the adverts in the latest entry on his blog.

"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."