In a US study three out of four previously infertile women who used Viagra became pregnant.

However, the doctor who carried out the study has warned that infertile women should not rush to take the drug hoping that it will solve their problems.

The drug only appears to work for women who have a specific problem - a thin lining of the uterus.

This medical condition prevents the embryo from attaching to the wall of the uterus.

Viagra works in men by increasing blood flow to the penis.

Doctors believe it may also act to increased blood flow to the uterus. This would increase the thickness of the uterus lining and boost the chances that the embryo could attach and develop successfully.

However, some experts fear Viagra may also damage the mother and the developing foetus.

In a statement the American Society for Reproductive Medicine said: "There are risks to Viagra use.

"Known side effects for women include high levels of nitric oxide in the womb, which can be both dangerous for the mother and toxic to developing embryos."

The research was carried out by Las Vegas doctor Geoffrey Sher, who ground up the Viagra tablets and made them into a vaginal suppository which the women inserted four times a day.

Dr Sher said further research was needed before Viagra could be considered as a fertility treatment.

Only a small proportion of infertile women suffer from a thin uterine lining.

Mr Derek Machin, a urologist and expert in Viagra based at University Hospital, Aintree, said the drug had originally been developed as a treatment for heart conditions.

He said: "As often happens with drugs they have new uses that you never suspect when they originally come out.

"Anything that will help people with infertility has got to be a jolly good thing."

Mr Machin said the drug was also being used to treat a condition known as acholasia. This is a spasming of the sphincter between the stomach and the gullet which results in food being vomited back into the mouth.

The fertility study will be published in the journal Human Reproduction.