The oestrogen cream, Oestriol, works by quadrupling the thin layer of keratin, a defensive protein, in the skin.

"By using keratin we can increase the body's natural defence ... and then the virus can't physically inject itself through that barrier to infect the cells underneath," Dr Pask said.

Circumcision provides up to 80 per cent protection from HIV and oestrogen would fill the gap, he said.

In uncircumcised men it is a living barrier against HIV.

"It's not a contraception ... but it is a living condom and a perfect protection against HIV," Prof Short said.

Forty million people have HIV worldwide.

Every year there are five million new infections and three million people die.

The treatment, details of which appeared today in the medical journal, PLoS ONE, published by the Public Library of Science, has worked in laboratory tests and will undergo clinical trials in Africa, the epicentre of the Aids epidemic.

"Mathematical models would predict that within say 50 or 60 years that the level of HIV in the world would be significantly reduced as an effect of this," Dr Pask said.

"The studies that they've shown on circumcision showed that within 50 to 60 years it would be less than half of what we have now, so it is a massive reduction.

"And then of course it would continue to drop exponentially from there until hopefully, if everybody in the world was to be circumcised or was to use the treatment, then eventually the epidemic would just completely disappear."

Dr Pask said it was a simple, cheap and effective guard against HIV that could be applied once a week in cultures where circumcision and safe sex were not practised.

But it does not protect against sexually transmitted infections.

Oestrogen is currently used to treat prolapse in woman and in small, external doses would have virtually no side effects in men.

It could eventually have applications in condoms and lubricants.