Any research into this area is to be encouraged

Terrence Higgins Trust spokeswoman

Scientists are not sure how it works to fight HIV, but think it binds to the virus and coats it in a substance like a thick paint, preventing it from attacking the human body.

A preliminary study has been completed in Thailand and South Africa, where women experienced no side effects.

Full-scale trials will involve 6,000 women.

Dr Helen Gayle, an adviser on HIV and Aids in the US, said: "This would allow women to take prevention into their own hands.

"It is very difficult for women in the world to negotiate safe sex and insist on their partner using condoms.

"And the reason HIV is spreading is not primarily because of women's risky behaviour.

"It's due to risky behaviour of their male partners."

Cautious welcome

Under a four-year study, one third of the women will be given the gel, one third a placebo and one third will not receive any, while all will be given condoms and taught about their use.

HIV and Aids charity, the Terrence Higgins Trust, has welcomed the development.

A spokeswoman said: "There isn't any evidence at the moment that the gel will work, but any research into this area is to be encouraged.

"We hope the trials go well."

The gel is usually used as a thickening agent in toothpaste, ice cream and baby milk.

Studies into the gel by the American Population Council's centre for Biological Research are being funded by the foundation set up by Microsoft founder Bill Gates.

Researchers are aware that a similar four-year study led by the United Nations Aids group on another gel, was unsuccessful.

The non-oxynol nine gel, was based on one already used in condoms as a spermicide.

Scientists found it not only failed to prevent the spread of HIV, it actually increased the number of infections among a group of prostitutes in Thailand and South Africa.