A DECISION by the Indian Supreme Court to reinstate a ban on gay sex represents a "significant step backwards for India" and violates international law, UN human rights chief Navi Pillay has said as she suggested the case should be reheard.

In a major blow to gay rights in the world's largest democracy, the Supreme Court on Wednesday threw out a 2009 ruling by a lower court that had decriminalised gay sex.

Ms Pillay said: "Criminalising private, consensual same-sex sexual conduct violates the rights to privacy and to non-discrimination enshrined in the International Covenant on Civil and Political Rights, which India has ratified.

"Yesterday's Supreme Court decision in this case represents a significant step backwards for India and a blow for human rights."

The top court ruled only India's government could change the law, deeming the Delhi High Court had overstepped its powers with the decision four years ago.

Ms Pillay voiced hope the Court might exercise its review procedure, in effect agreeing to rehear the case before a larger panel of judges.

She said this would provide an opportunity for judges to reconsider the Supreme Court's decision.