If you're a guy who enjoys sitting through long traffic jams because of your heated car seat, you never have to buy another condom ever again

Heated car seats may keep drivers warm on cold mornings - but they could also lower a man's sperm count, scientists claim.

A study found the electric-generated heat emitted by some driver's seats raises body temperature to a point that interferes with the production of healthy sperm.

It follows evidence from previous studies that men who sit in a car for more than three hours a day, or who work as long-distance lorry drivers or long-haul pilots, could be putting their fertility at risk.

The ideal temperature for sperm production is around 35C to 36C (95F to 97F), just below the average core body temperature of 37C (99F).

If temperatures are much higher - because, for instance, men wear tightly-fitting trousers or underwear, have long, hot baths or spend too long sitting down - sperm counts can fall dramatically.

Some fertility experts believe changes in fashion and lifestyles are partly to blame for falling sperm counts in recent decades.

To test the effects of car seat warmers, researchers at the University of Giessen, Germany, fitted temperature sensors to 30 healthy volunteers who then sat on a car seat fitted with a warmer for 90 minutes.

Within an hour, temperatures had risen from to an average of 37.3C.

One man recorded a temperature of 39.7C (103.5F) New Scientist magazine reported yesterday.

Sitting on an unheated seat also led to slightly raised temperatures of around 36.7C (98.1F).

While the researchers did not test the quality or quantity of the volunteers' sperm, they are worried the effects of car seat heaters may pose a greater risk to fertility than merely sitting for long periods.

Researchers in Holland compared the sperm counts of men who wore tight-fitting underpants for six months and found it fell compared to when they wore loose undergarments.