India's western state of Maharashtra has introduced a ban on beef so strict that even possession could lead to five years in jail.
The country's Hindu majority considers cows sacred and several states already ban their slaughter.
But the latest measures in Maharashtra - home to India's commercial centre Mumbai - go even further, making sale or possession of beef an offence punishable by a five-year jail term or a 10,000 rupee ($A205) fine.
The Indian Express newspaper says the measures became law after President Pranab Mukherjee gave his assent to a legal amendment passed by the state parliament two decades ago.
The measures include a ban on the slaughter of bulls and bullocks, hitherto legal with a vet's certificate, although it will still be legal to slaughter buffalo.
Maharashtra's chief minister Devendra Fadnavis tweeted his thanks to the president, saying "our dream of ban on cow slaughter becomes a reality now".
Right-wing Hindu groups in India have long demanded a complete ban on the slaughter of all cattle, citing religious scriptures.
The main players in the beef industry are Muslims, the country's largest religious minority, who make up some 13 per cent of India's 1.25 billion population.
Maharashtra state is ruled by Prime Minister Narendra Modi's Hindu nationalist Bharatiya Janata Party in alliance with the far-right Shiv Sena party.
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