A gas behind the smell of rotting eggs has been found to play a key role in giving men erections. Scientists believe the discovery could lead to the development of a male impotence drug to rival Viagra.

The whiff of hydrogen sulphide -- a gas not traditionally associated with lovemaking -- accompanies the biological degradation of sulphur-containing substances. It also belches from the exhausts of cars fitted with catalytic converters.

Prior to sexual intercourse, quantities of hydrogen sulphide are released within nerve cells of the penis which control the engorgement of the male organ with blood to stiffen it.

Scientists believe the discovery could lead to the development of a new class of drugs to combat erectile dysfunction by affecting a different biochemical pathway to the one targeted by Viagra. The research is published in the journal Proceedings of the National Academy of Sciences.