A new version of the "prohibited" sign (the red circle with a line through it) has arrived in service stations in many parts of the US. To petrol-pumping Americans, the decision by some oil companies to ban customers' use of cellphones while they fill their tanks has come as a big surprise.

"It's so nineties," said Trisha, who works for Compaq, the computer maker, in human resources, while filling up at a Chevron station on the corner of Dallas and Waugh streets, close to downtown Houston. She found it hard to believe that her cellphone could cause an explosion. Just as surprising, she said, the ban "assumes that everyone has a cellphone".

Before the end of the year, oil companies including BP-Amoco and Chevron are going to put up, or amend, signs at their service stations warning customers not to use their cellular phones or other personal electronic devices, such as pagers or laptop computers, while refuelling. Signs will warn people to turn off these devices and inform them of the potential dangers.

While there is no evidence to support their concern, oil companies are worried that the energy from a cellphone battery might cause petrol vapours to ignite. Although they concede the danger is slight, they say they want to err on the side of caution.

"We live in a litigious society, and failure to warn people is probably going to put us at risk," said Mr Howard Miller, a spokesman for BP-Amoco."

Oil companies already ban the use of cellphones at service stations in some European countries, but to some Americans, the ban is folly. "There are more important issues to address than the potential to blow up a gas station from a cellphone," said Jim, an industrial sales engineer, filling up his tank at an Exxon station off the busy I-10 freeway.

"It's another issue for the government to encroach on our liberties."