Married men who have unprotected sex with prostitutes are the biggest culprits when it comes to spreading the AIDS virus in India, according to a study funded by the Bill & Melinda Gates Foundation.

About 610,000 Indians contracted HIV last year, increasing the overall number to about 4.5 million, it said. That is the second-highest total in the world after South Africa.

The study, which used central and state government data, said business, men in service sector and hotel employees contracted the virus in four of the six most affected states.

"The spread from the high-risk behaviour groups to the general populace is another key indicator that HIV/AIDS in India has reached epidemic proportions," the study said.

Sex workers' clients, "particularly married males, act as the bridge groups aiding (the spread)… into the general population".

The study, citing India's National Aids Control Organisation, found that more than 85 per cent of India's HIV cases are caused by unsafe sex.

Last month, the foundation pledged $200 million for an HIV-prevention programme in India.