Impotent men who take a new drug for the condition may be able to have sex up to 24 hours later, claims the maker.

Cialis, a medication trying to gain a foothold in the lucrative anti-impotence market created by Viagra, is being tested by doctors.

Pharmaceutical firm Eli Lilly wants to be able to market Cialis in the US from the second half of this year.

Their trial involved 223 men with moderate to severe impotence, some of whom were given Cialis, and the rest a dummy placebo.

Conditions for which Viagra can be prescribed

Diabetes Multiple sclerosis Parkinson's Disease Poliomyelitis Prostate cancer Prostatectomy Radical pelvic surgery Renal failure treated by dialysis or transplant Severe pelvic injury Single gene neurological disease Spinal cord injury Spina bifida

While the drug group were statistically more likely to achieve erection, they were also more likely to have successful second sexual encounters during a 24-hour period.

Dr Harin Padma-Nathan, a professor of urology at the University of Southern California, who led the study, acknowledged that other impotence drugs had not been tested for this longevity of effect.

However, he said: "We saw a huge clinical difference within 24 hours in patients taking Cialis and many of the men indicated a sense of freedom and spontaneity."

Other trials have also suggested that the drug is effective against some forms of impotence.

It is estimated that more than 7m men worldwide have taken the original impotence drug, Viagra, since it came onto the market in 1998.

Almost �400m has been spent paying for the treatment.

It is not freely available on the NHS, although men with a range of conditions which cause impotence as a side-effect are given a fixed number of pills on prescription.

Approximately 300,000 NHS prescriptions for the drug are handed out every year.