But a study suggests blasting a man's most vulnerable area with high frequency sound waves may be the ideal form of male contraception.

Scientists conducted experiments which showed that ultrasound effectively cut sperm counts in rats. The equivalent outcome would result in reversible infertility in humans.

Study leader Dr James Tsuruta, from the University of North Carolina, US, said: "Unlike humans, rats remain fertile even with extremely low sperm counts. However, our non-invasive ultrasound treatment reduced sperm reserves in rats far below levels normally seen in fertile men. Further studies are required to determine how long the contraceptive effect lasts and if it is safe to use multiple times."

The perfect male contraceptive would be cheap, reliable, reversible, long-acting and have few side effects. The ultrasonic "sperm zapping" treatment potentially appears to tick those boxes, although it remains to be proven with more research.

Dr Tsuruta's team found that rotating a three megahertz high frequency ultrasound beam around rats' testes wiped out the germ cells that produce sperm. Best results were seen from two sessions lasting 15 minutes with the testes warmed to 37C.

A first attempt to use ultrasound as a male contraceptive was reported 40 years ago. Several prostate cancer patients who were due to have their testicles removed underwent the treatment, which resulted in a "dramatic loss of germ cells".

"These men reported that the procedure was pain-free, only creating a gentle feeling of warmth," said the authors of the new study writing in the journal Reproductive Biology and Endocrinology.

However, the idea was not pursued and the equipment used is now outdated and unavailable. The new research was intended as a starting point to see if modern ultrasound devices used for physical therapy can be used as a male contraceptive.

The scientists wrote: "The non-invasive nature of ultrasound and its efficacy in reducing sperm count make therapeutic ultrasound a promising candidate for a male contraceptive.. Our results clearly show that therapeutic ultrasound treatment depleted developing germ cells from the testis and subsequently decreased the size of sperm reserves."