India's ruling Hindu nationalist party approved a decree in Uttar Pradesh on Tuesday laying out prison terms for anyone compelling others to convert their faith or luring them into these conversions through marriage, officials said.
The move follows a campaign by hardline Hindu groups against some interfaith marriages that they describe as "love jihad", Muslim men engaging in a conspiracy to turn Hindu women away from their religion by seducing them.
Critics said the unlawful conversion order approved by the cabinet of Uttar Pradesh, run by Prime Minister Narendra Modi's BJP, was aimed at further alienating India's 170 million Muslims by painting them as aggressors plotting to weaken Hindus.
Little data exists to show how many interfaith marriages took place in the state, the first in the country to bring in such legislation.
Uttar Pradesh cabinet minister Siddharth Nath Singh said prison terms of up to five years were necessary to stop unlawful conversions and provide justice to women who have suffered from them.
Under the new law, a man and woman belonging to different religions will have to give two months' notice to the district magistrate before they get married and they will be allowed to tie the knot if there are no objections.
Hindus makes up 80 percent of India's 1.3 billion population. But hardline groups accuse political parties of appeasing minority groups such as Muslims for votes and in recent years have stepped up a campaign for a Hindu-first India.
Nusrat Jahan, a member of the national parliament from a regional group most active in the neighbouring state of West Bengal, told NDTV television news channel the decree smacked of politics - even though regional elections are at least a year away.
"This is just another agenda before the elections. There is nothing like 'love jihad' that exists. People can make their own decisions," she said.