Derek Machin, a consultant urologist in Liverpool, said women will be using the anti-impotence drug within months of it being available on the NHS.

Urologists believed the drug, which is claimed to give middle-aged men the libido of 20-year-olds, could also work for women.

The drug's manufacturer, Pfizer, has confirmed it is carrying out trials on women who have a problem reaching orgasm.

Until now, the focus has only been on male impotence. Viagra, which was initially developed to fight high blood pressure, makes the blood flow to the penis and helps men get and sustain an erection.

It is estimated that up to one in 10 men in the UK are impotent. The numbers of women who are unable to have an orgasm due to clitoral erection is unknown since women do not traditionally complain about sexual problems.

Side effects

Mr Machin said: "By and large, women don't come forward and complain about problems. A lot of them don't realise there is a problem or that there is anything they can do about it."

Doctors warn against possible Viagra side effects

He warned that it would be difficult to stop the use of the drug and said it was unclear what the side effects could be for women. However, he said he would advise pregnant women not to take it.

The US Food and Drug Administration is investigating the deaths of around 20 men in connection with the drug and Pfizer has warned people not to mix it with nitrate-based drugs.

Viagra is expected to be licensed for sale in Europe in the autumn.

Doctors say it should not be freely available on the NHS because, at around £6 a pill, it could bankrupt the NHS. They say it should be restricted to those who have a clinical need for it.

But Mr Machin said it would be difficult to stop people getting hold of the drug on the black market.

"Once Viagra is available, it will be used by both sexes. It is inevitable and word of mouth is the best recommendation. If women find it works, that is how it will spread," he said.

Female impotence

Viagra's manufacturer, Pfizer, announced on Tuesday that it is carrying out trials on women to see what the effects are on them. Several British woman are thought to be involved in the European trial.

A spokesman for Pfizer said: "We are probably where we were 10 years ago with male erectile dysfunction." He added that results of the trials were not expected until at least next year.