From Hitchcock's The Birds to The Usual Suspects, it has been one of the staple cliches of Hollywood: the cigarette butt tumbling in slow motion into a pool of petrol unleashing a conflagration.

But if you find yourself tied up and doused in petrol don't worry if all your assailant has is a lighted cigarette: scientists have proved you won't end up as a human fireball.

"On the face of it it's a pretty simple problem," said Richard Tontarski, an expert in forensic fire at the Bureau of Alcohol, Tobacco and Firearms research laboratory in Beltsville, Maryland. Cigarettes burn at around 700C (1,292F) and the ignition temperature of petrol is 246C. "But it just isn't that simple," he said.

He began looking into the problem because arson suspects frequently claim a petrol fire was started by accident. "The person claims, 'I accidentally threw gasoline on my girlfriend, she was smoking and she burst into flames'," he said.

To find out whether this was possible, he and colleagues experimented. They dropped burning cigarettes into trays of petrol. They sprayed a fine mist of petrol at a lighted cigarette. They even used a vacuum device to produce the higher temperature (900-950C) of a cigarette being sucked. In more than 2,000 attempts the petrol did not ignite.

Dr Tontarski can only speculate why. The layer of ash on the tobacco, perhaps, or the petrol vapour convected away from the hottest part of the cigarette.

So is he in touch with Hollywood to demand it drops the explosive movie cliche? "Actually they are pretty well aware of it. They don't care."