A Christian bus driver in Southampton has refused to take to the road in a vehicle emblazoned with an advertisement for a new campaign promoting atheism.

Ron Heather, 62, told managers at First Bus that his beliefs would not permit him to drive a bus carrying the message: "There's probably no God. Now stop worrying and enjoy your life."

The advertisements, initially suggested in a guardian.co.uk article by the comedy writer Ariane Sherine, first appeared on London buses and are now being placed on 800 vehicles around the country in a campaign organised by the British Humanist Association.

First Bus has made arrangements to accommodate Heather's beliefs, attempting to ensure he does not drive any of the buses carrying the advertisement.

"When I first saw the bus last Saturday I was shocked," Heather said. "I was just about to board and there it was staring me in the face. My first reaction was horror. I'd heard about this silly campaign in London but I had no idea it was coming to Southampton. I had certainly hoped they were not coming here because I didn't want to make a stand.

"I was in a dilemma but I felt strongly I couldn't drive that bus and so I went up to my inspectors and told them there was no way I could drive it. They said they didn't have another one, so I thought I'd better go home."

First Bus had been "very good" and was doing its best to ensure he did not have to drive any of the buses, he said. However, if the atheist campaign continued beyond its scheduled run to the end of February "I will seriously consider giving up my job".

A spokesman for First Bus said the company took no stance on any legal advertisements placed on its vehicles but recognised the strength of Heather's feelings and was "doing what we can to accommodate his request not to drive the buses concerned".

The spokesman added: "Mr Heather accepts though that he may need to drive one of these buses if no other vehicle is available for him."

The £140,000 advertising campaign, backed by a number of leading atheists and secularists, among them Richard Dawkins, was launched on January 6, with the money raised by a fundraising drive.

Sherine raised the idea in a comment article in June last year, after seeing advertisements espousing Christian views on London buses. She argued that she found the faith's threat of hell and damnation alarming.

Within days of the adverts appearing, more than 100 people complained to the Advertising Standards Authority, among them Stephen Green, head of the campaign group Christian Voice.

"It is given as a statement of fact and that means it must be capable of substantiation if it is not to break the rules," he said. "There is plenty of evidence for God, from people's personal experience, to the complexity, interdependence, beauty and design of the natural world.