The gel, spiked with the AIDS drug tenofovir, cut the risk of HIV infection by 50 percent after one year of use and 39 percent after two-and-half years, compared to a gel that contained no medicine.

To be licensed in the US, a gel or cream to prevent HIV infection may need to be at least 80 percent effective, Fauci said. That might be achieved by adding more tenofovir or getting women to use it more consistently. In the study, women used the gel only 60 percent of the time; those who used it more often had higher rates of protection.

The gel also cut in half the chances of getting HSV-2, the virus that causes genital herpes. That's important because other sexually spread diseases raise the risk of catching HIV.

Even partial protection is a huge victory that could be a boon not just in poor countries but for couples anywhere when one partner has HIV and the other does not, said Dr Salim Abdool Karim, the South African researcher who led the study. In the US, nearly a third of new infections each year are among heterosexuals, he noted.

Countries may come to different decisions about whether a gel that offers this amount of protection should be licensed. In South Africa, where one in three girls is infected with HIV by age 20, this gel could prevent 1.3 million infections and 826,000 deaths over the next two decades, he calculated.

He will present results of the study on Tuesday at the International AIDS Conference in Vienna. The research was published online on Monday by the journal Science.

"We now have a product that potentially can alter the epidemic trends ... and save millions of lives," said Dr Quarraisha Abdool Karim, the lead researcher's wife and associate director of the South African program that led the testing.

It's the second big advance in less than a year on the prevention front. Last fall, scientists reported that an experimental vaccine cut the risk of HIV infection by about 30 percent. Research is under way to try to improve it.

If further study shows the gel to be safe and effective, WHO will work to speed access to it, said its director-general, Dr Margaret Chan.

The gel is in limited supply; it's not a commercial product, and was made for this and another ongoing study from drug donated by California-based Gilead Sciences Inc., which sells tenofovir in pill form as Viread. If further study proves the gel effective, a full-scale production system would need to be geared up to make it.

The study tested the gel in 889 heterosexual women in and near Durban, South Africa. Researchers had no information on the women's partners, but the women were heterosexual and, in general, not in a high-risk group, such as prostitutes.

Half of the women were given the microbicide and the others, a dummy gel. Women were told to use it 12 hours before sex and as soon as possible within 12 hours afterward.

At the study's end, there were 38 HIV infections among the microbicide group versus 60 in the others.

The gel seemed safe -- only mild diarrhea was slightly more common among those using it. Surveys showed that the vast majority of women found it easy to use and said their partners didn't mind it. And 99 percent of the women said they would use the gel if they knew for sure that it prevented HIV.

This shows that new studies testing the gel's effectiveness without a placebo group should immediately be launched, said Salim Abdool Karim. The only other study testing the gel now compares it to placebo and will take a couple more years to complete.

The study was sponsored by the Centre for the AIDS Programme of Research in South Africa, or CAPRISA; Family Health International; CONRAD, an AIDS research effort based at Eastern Virginia Medical School; and the U.S. Agency for International Development, or USAID.

Gilead has licensed the rights to produce the gel, royalty-free, to CONRAD and the International Partnership on Microbicides for the 95 poorest countries in the world, said Dr. Howard Jaffe, president of the Gilead Foundation, the company's philanthropic arm.

The biggest cost of the gel is the plastic applicator -- about 32 cents, which hopefully would be lower when mass-produced, researchers said.

Mitchell Warren, head of the AIDS Vaccine Advocacy Coalition, a nonprofit group that works on HIV prevention tools, said the study shows a preventive gel is possible.

"We can now say with great certainty that the concept has been proved. And that in itself is a day for celebration," he said.

In 2002 the Convention on Biological Diversity made a promise to slow the rate of biodiversity loss around the globe by 2010.

But the United Nations body has now admitted in the leading journal Science that the target will not be met.

Stuart Butchart, of the UN Environment Programme World Conservation Monitoring Centre, said if anthing the situation has got worse with animal populations down by a third since 1930.

He blamed the growing human population that is driving the development of cities, pollution, climate change and the intensification of agriculture.

Dr Butchart studied 31 indicators submitted by conservation bodies around the world including wild birds numbers, fish stocks, coral reefs, rainforests and the state of wild animal populations.

Key species such as bluefin tuna, tigers, the Pacific walrus and the monarch butterfly continue to decline.

"Our analysis shows that governments have failed to deliver on the commitments they made in 2002: biodiversity is still being lost as fast as ever, and we have made little headway in reducing the pressures on species, habitats and ecosystems," he said.

With species extinctions running at about 1,000 times the "natural" or "background" rate, some biologists contend that the world is in the middle of the Earth's sixth great extinction – the previous five stemming from natural events such as asteroid impacts.

It has been known for some time that the 2010 target to halt biodiversity loss would not be met, but the article in Science makes the failure official.

It is a particular embarrassment as this year is the UN International Year of Biodiversity.

The UK Government failed to stop the decline of species like hedgehogs, house sparrows and dormice in its own backyards and other developed nations have also failed to stop species loss despite spending millions on nature reserves and new legislation to stop persecution.

But Dr Butchart said the UN could repair the situation by drawing up a more ambitious legally-binding treaty at a meeting of the CBD later this year that would commit countries to halting biodiversity loss once and for all.

"Our data show that 2010 will not be the year that biodiversity loss was halted, but it needs to be the year in which we start taking the issue seriously and substantially increase our efforts to take care of what is left of our planet," he added.

:: The world’s largest business and biodiversity conference and exhibition will be held in London later this year.

The first 'Global Business of Biodiversity Symposium,’ held by the UK Government and the European Commissioner for Environment, will bring together business and environmentalists to discuss the best way to stop.

By Louise Gray, Environment Correspondent; The Telegraph