Motorists who do not use screenwash for their windscreen wipers risk getting potentially deadly legionnaires' disease, a study reveals.

The warning comes after health experts discovered professional drivers were five times more likely to be infected with the dangerous bug, which is found in warm, stagnant water.

Drivers are now being urged to add screenwash to their wiper water after traces of the legionella bacterium were found in one in five cars that did not have the additive - but in no cars that did.

It is feared that around 20% of legionnaires' disease cases could arise from this type of exposure.

The findings come from a Health Protection Agency-led study, which looked at why people at the wheel were more likely to be infected. Most at risk were found to be those driving a van, people who drive through industrial areas, and people who often had the car window open.

But the "most intriguing" higher-risk group was drivers not using screenwash - which kills off the legionella bug, the study authors found.

They said: "Not adding screenwash to windscreen wiper fluid is a previously unidentified risk factor and appears to be strongly associated with community acquired sporadic cases of legionnaires' disease.

"We estimated that around 20% of community acquired sporadic cases could be attributed to this exposure." The report - published in the European Journal of Epidemiology - added: "This simple public health advice may be of worldwide relevance in reducing morbidity and mortality from Legionnaires' disease."

Stagnant, warm water is a breeding ground for the legionella bacterium, which causes a potentially fatal lung infection. It is contracted when small droplets of contaminated water are breathed in, and cannot spread from person to person.

Last year there were 345 cases in England and Wales - although some infections were caught overseas. An estimated 10% of people who contract legionnaires' disease will die from complications arising from infection.

Bacteriology expert Professor Hugh Pennington told the BBC: "This is a bug which lives in the environment and will take advantage of warm water systems that are not cleaned out.

"Windscreen fluid stops the bug from growing. If you can prevent it with something this simple then it's a no brainer really."