International drug company Merck has halted trials on an HIV vaccine that was regarded as one of the most promising in the fight against Aids.

Merck stopped testing the vaccine after it was judged to be ineffective.

In trials, the vaccine failed to prevent HIV infections among volunteers who were at risk of catching the virus, including gay men and sex workers.

Merck had previously expressed high hopes for the drug, which it spent 10 years developing.

Merck's international trial, called Step, began in 2004 and involved 3,000 HIV-negative volunteers from diverse backgrounds, between the ages of 18 and 45.

Merck said that 24 of 741 volunteers who got the vaccine became infected with HIV, the virus that causes Aids.

Out of a group of 762 volunteers who were given a dummy version of the jab, 21 became infected with HIV.

An independent monitoring panel recommended discontinuing the vaccination of volunteers, saying the trail was headed for failure.

Most of the volunteers were at high risk of HIV infection.

They were repeatedly given advice about how to practise safe sex, according to Merck.

The vaccine contained a common cold virus loaded with copies of three HIV genes.

The hope was that exposure to the genes would prompt an immune response in the body so that cells containing HIV virus would be recognised and destroyed.

'Today is a very sad day for the industry because Merck's vaccine had shown an ability to turn on the immune system, which gave many people optimism it would work,' said Sarah Alexander, from the HIV Vaccine Trials Network.

Doctors have said a preventative vaccine would be the best way to control the spread of HIV.