The gas responsible for the foul odour of rotten eggs could hold the key to a new impotence drug, experts believe.

A team from the University of Naples Federico II in Italy found the release of hydrogen sulphide in the penis plays a role in creating an erection.

Researchers said the study could lead to an alternative to Viagra, the Proceedings of the National Academy of Science journal reported.

Erection problems affect about one in 10 men.

It has been suggested that hydrogen sulphide, which is also found in car exhaust fumes, helps relax nerve cells in the penis to stimulate blood flow.

The process mirrors the earlier discovery of the role played by nitric oxide on a slightly different area of the penis, which eventually led to the development of Viagra.

The researchers tested this theory by injecting the gas into intact erectile tissue from eight men who had undergone sex changes surgery as well as carrying out tests on rats.

Lead researcher Professor Giuseppe Cirino said it seemed certain that the gas was partly responsible for the erection process and this could lead to a new drug.

He added: "This may help to unravel the complex mechanisms underlying the physiology of human penile erection and may lead to the development of therapeutic approaches in the treatment of erectile dysfunction and sexual arousal disorders."

Dr Graham Jackson, chairman of the Sexual Dysfunction Association, said the development of a new impotence drug would be welcome.

"There is certainly a need for an alternative to Viagra. It is only about 60% effective in people with diabetes and 80% to 85% effective for the general population."