About 800 buses bearing the slogan "There's probably no God. Now stop worrying and enjoy your life" set off on Britain's roads on Tuesday in an atheist campaign responding to a set of Christian ads.
The campaign, which will also see slogans plastered across London's underground system, was paid for by more than £140,000 ($A288,000) in public donations, the British Humanist Association said.
It was the brainchild of comedy writer Ariane Sherine, 28, who objected to the Christian adverts on some London buses that carried an internet address warning that people who rejected God would spend eternity in "torment in hell".
She sought £5 ($A10.30) donations towards a "reassuring" counter-advertisement and won support from the BHA and atheist campaigner Professor Richard Dawkins.
"You wait 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," Sherine said as the buses were launched across Britain on Tuesday.
She added: "I am very glad that we live in a country where people have the freedom to believe in whatever they want."
From next Monday, quotations from Albert Einstein - "I do not believe in a personal God and have never denied this but have expressed it clearly" - and others will also be displayed on advertising hoardings in the London underground.