Bill and Melinda Gates have come off the political fence and publicly backed key causes of Aids campaigners, criticising the abstinence policies beloved of the US government and calling for more rights for women and help for sex workers.

Making the keynote speech of the opening session of the 16th International Aids Conference in Toronto, on Sunday, the Microsoft billionaire and his wife, who have previously largely confined themselves to discussing and funding non-political scientific research, spoke with passion and commitment about the social changes necessary to stop the spread of HIV/Aids.

The "ABC" programme - abstinence, be faithful and use a condom - had saved many lives, Mr Gates told the conference of more than 20,000 delegates. But, departing from his government's line, he added that for many at the highest risk for infection, ABC had its limits.

"Abstinence is often not an option for poor women and girls, who have no choice but to marry at an early age. Being faithful will not protect a woman whose partner is not faithful. And using condoms is not a decision that a woman can make by herself; it depends on a man.

"We need to put the power to prevent HIV in the hands of women. This is true whether the woman is a faithful married mother of small children or a sex worker trying to scrape out a living in a slum. No matter where she lives or what she does, a woman should never need her partner's permission to save her own life."

The Gates Foundation is funding research into microbicides - gels or barrier creams a woman can use before sex that will kill HIV. An update on the research will take place during the week-long conference.

Mrs Gates called for an end to the stigma affecting those with HIV. "Stigma makes it easier for political leaders to stand in the way of saving lives," she said, in a direct attack on some African leaders whose reservations over condoms are encouraged by the US Christian fundamentalist right's belief that abstinence from sex is the answer.

"In some countries with widespread Aids epidemics, leaders have declared the distribution of condoms immoral, ineffective or both. Some have argued that condoms do not protect against HIV, but in fact help spread it. This is a serious obstacle to ending Aids. In the fight against Aids, condoms save lives. If you oppose the distribution of condoms, something is more important to you than saving lives."

She also criticised African governments whose officials accompanied the couple on their fact-finding trips, such as the recent one in July, up until the point that they went to see sex workers. "At that point, it can become too politically difficult to stay with us and our official hosts often leave," she said.