By Martin Beckford, Religious Affairs Correspondent
Organisers originally hoped to put the message on just a handful of London buses, as an antidote to posters put up by religious groups which they claimed were "threatening eternal damnation" to non-believers.
But after the campaign received high-profile support from the prominent atheist Prof Richard Dawkins and the British Humanist Association, the modest £5,500 target was met within minutes and more than £140,000 has now been donated since the launch in October.
Enough money has now been raised to place the message – "There's probably no God. Now stop worrying and enjoy your life" – on 200 bendy buses in the capital for a month, with the first ones taking to the streets .
A further 600 buses carrying the adverts will be seen by passengers and passers-by in cities across England, Wales and Scotland, from Aberdeen and Dundee to York, Coventry, Swansea and Bristol.
In addition, two large LCD screens bearing the atheist message have been placed in Oxford Street, central London, while 1,000 posters containing quotes from well-known non-believers will be placed on Underground trains for two weeks starting on Monday.
They feature lines doubting the existence of God, and celebrating the natural world, written by Albert Einstein, Katharine Hepburn, Douglas Adams and Emily Dickinson.
It is the first ever atheist advertising campaign to take place in Britain, and similar adverts are now also running on public transport in America and Spain.
Ariane Sherine, a writer who first thought of the atheist bus adverts, said: "You wait ages for an atheist bus, then 800 come along at once. I hope they will brighten people's days and make them smile on their way to work."
The campaign has even been welcomed by religious groups for increasing the profile of debate about faith, and although there was tight security outside the launch event by the Royal Albert Hall, the campaigners have not received any threats from fundamentalists.
Paul Woolley, director of Theos, a theology think tank which donated £50 to the cause, said: "The posters will encourage people to consider the most important question we will ever face in our lives."
Some atheist supporters of the campaign were disappointed that the wording of the adverts did not declare categorically that God does not exist, although there were fears that this could break advertising guidelines.
Prof Dawkins, the renowned evolutionary biologist and author of The God Delusion, said: "I wanted something stronger but with hindsight I think it's probably a good thing because it makes people think. It's just food for thought – people will have conversations in pubs when they see these buses."
Hanne Stinson, chief executive of the British Humanist Association, said the adverts were "overwhelmingly positive" and were intended to reassure agnostics and atheists that there is nothing wrong with not believing in God.