Scientists appear to have made a major breakthrough in efforts to prevent Aids, as a new study found that treatment can prevent sexual transmission of the virus.

The study, published in The Lancet medical journal this week, examined nearly 1,000 gay male couples who had sex without condoms, in which one partner was HIV positive and treating the condition with antiretrovirals. It found no cases of transmission over the space of eight years.

Such is the success of the medicine, researchers said, that if everyone with HIV was fully treated, there would be no further infections.