Two British doctors investigated the phenomenon after reading of a middle-aged patient who had uncontrollable sneezing fits when he thought of sex.

They unearthed evidence, via internet chat rooms, of 17 others - of both sexes - with the same problem.

He said: "It certainly seems odd, but I think this reflex demonstrates evolutionary relics in the wiring of a part of the nervous system called the autonomic nervous system.

"This is the part beyond our control, and which controls things like our heart rate and the amount of light let in by our pupils.

"Sometimes the signals in this system get crossed, and I think this may be why some people sneeze when they think about sex."

He said during arousal the autonomic nervous system fires off signals to trigger changes in the genitals of both men and women.