The company that developed the vaccine, VaxGen, said it hoped to inoculate several thousand volunteers in the United States, Canada and Thailand over a three-year period.

The company, based in California, said the tests would help determine whether the product could be licensed.

But several researchers into Aids said they doubted the new vaccine, called Aidsvax, would work.

Recent research by a group of American scientists suggests that the HIV virus, which causes Aids, is capable of resisting all the drugs approved to treat the disease in the United States.

The new vaccine is intended to stimulate the body to create antibodies that would stop the virus from attaching to receptors on the white blood cells it attacks.

Experts agree that, in the long term, a vaccine is the only answer to HIV.

Cocktails of strong drugs can keep the virus at bay, but they are very expensive and have severe side effects.

Some Aids researchers have expressed concern that a vaccine would partially protect patients, and that patients, believing themselves to be protected, would engage in risky behaviour such as unprotected sex or sharing needles.