The £130,000 raised by the British Humanist Association has paid for a four-week bus campaign from today and two weeks of exposure on the tube from Monday, both through CBS Outdoor.
Two hundred bendy buses in London and 600 in regions across the UK, including the West Midlands and Glasgow, will carry an ad for the BHA which says "There's probably no god. Now stop worrying and enjoy your life".
The ads were funded by donations to the BHA following the suggestion of a bus campaign by comedy writer Arianne Sherine on The Guardian's Comment is Free blog last June.
Sherine, who had the idea after seeing a Christian bus ad linking to a website telling non-Christians they were condemned to Hell for not accepting the word of Jesus, appealed for £23,400.
The copy on the 1,000 additional tube cards varies from the bus campaign.
Four different ads each carry a quotation from well-known atheists such as scientist Albert Einstein and actress Katherine Hepburn.
One from the nineteenth century poet Emily Dickinson reads: "That it will never come again is what makes life so sweet."
Sherine's idea received early and continuing support from high profile atheist, academic and writer Professor Richard Dawkins, who donated £5,500 and appeared in media interviews today to publicise the event.