The archsceptic professor Richard Dawkins today launched Britain's first atheist campaign posting the message: "There's probably no God. So stop worrying and enjoy your life" on the side of 800 British buses.

The posters – launched in conditions cold enough to freeze a presumably non-existent hell over – co-incided with the Christian feast of the Epiphany, when according to tradition three magi from the East presented the baby Jesus with gifts of gold, frankincense and myrrh.

Dr Dawkins, speaking at the launch in Central London, said he would have rather not had the word "probably" in the advertisement. He said the existence of God was about as likely as that of the tooth fairy.

Organisers of the four-week campaign said they had included the word "probably" because they did not want to be dogmatic in the way that so many religious leaders are.

The campaign was conceived by Ariane Sherine, a comedy writer who dreamed of just one bus going around London with an atheist slogan after she saw a similar advertisement for Christianity on the back of a bus.

She was upset to click on the web link it suggested and be directed to a site prophesying a future of hellfire and brimstone for all non-Christians. She determined to do what she could to offer non-believers a more life-affirming message and wrote about it in an online forum.

Her campaign sparked a massive and instant response and was taken up by the British Humanist Association and the scientist and Professor Dawkins, who pledged £5,500 of his own cash.

The organisers were astounded when with 24 hours of launching a fundraising campaign to get one bus on the road, their target of £5,000 was exceeded by thousands.

Within four days more than £100,000 had poured in, mainly in small amounts such as £5 and £10 from individuals. To date the campaign has raised £135,000 and Britain's atheists and agnostics are still contributing, in spite of the recession.

This means the number of buses and the length of the campaign were both increased beyond the organisers' most ambitious prayers.

Ms Sherine said she had been amazed and delighted by the response which had been totally unexpected. She said: "You wait for ages for an atheist bus, then 800 come along at once. I hope they'll brighten people's days and make them smile on their way to work."

Hanne Stinson, chief executive of the British Humanist Association, said: "The incredible response to the Atheist Campaign shows just how many atheists out there have been looking for a voice. Now that the buses are rolling out across the country, I feel sure that everyone with non-religious beliefs who spots one of these buses on the streets will be delighted to see what this amazing appeal has achieved."