Researchers have found that a combination of two experimental HIV vaccines cuts infection rates by one third.

The seven-year trial, carried out in Thailand on 16,000 people, is the largest vaccine trial ever carried out. The two vaccines had been tested previously, but had not yielded results when used separately.

The volunteers, all of whom were HIV-negative at the beginning of the trial, were men and women aged between 18 and 30.

Half were given the combination of vaccines, while half were given a placebo. They were then tested for HIV every six months for three years.

It was found that while 74 people in the control group contracted HIV, 51 of those in the vaccine group were infected – a 31.2 per cent drop.

The research was carried out by the US Army and the Thai Ministry of Public Health. HIV groups have been quick to welcome the development.

The International AIDS Vaccine Initiative (IAVI) described it as a "significant scientific achievement".

IAVI president Seth Berkley said: “It’s the first demonstration that a candidate AIDS vaccine provides benefit in humans. Until now, we’ve had evidence of feasibility for an AIDS vaccine in animal models. Now, we’ve got a vaccine candidate that appears to show a protective effect in humans, albeit partially.”

Deborah Jack, chief executive of the National AIDS Trust, said: “These vaccine trial results are very good news and represent a major breakthrough as it is the first time a vaccine candidate has shown real benefit in human trials. Ultimately vaccines are the most effective way by far of tackling serious infectious disease. And with over 2 million new HIV infections a year this option is desperately needed. Obviously there is much more work to do with these promising findings, but they justify the continuing investments and efforts of the international community, including the UK government, to develop a vaccine.”

Lisa Power, head of policy at Terrence Higgins Trust, said: "This is very good news for the future. While this trial only protected people 30 per cent of the time, it gives us a good idea of where to concentrate our research in the future. For now, of course, the best protection if you're having sex is still a condom."