Dr Bruno Debenois of the WHO on the benefits of iodised salt

The World Health Organisation and the United Nations Children's Fund recommended the move, which has just taken effect.

They say iodised salt provides the minute doses needed to combat the iodine deficiency to which tens of millions of Indians are said to be prone.

Salt producer and campaigner against iodising, Prabash Joshi

The deficiency can cause birth defects, mental retardation and goitre.

Experts say that iodizing salt is cheap, easy to do and effective.

A salt worker collects salt from the sea to dry in salt pans in north Bombay

But campaigners in Orissa and Rajasthan say the directive will put small salt producers out of business and force the poor to pay up to 12 times more for the staple commodity, even in areas where there is already iodine in the diet.

There have even been warnings of iodine overdoses, a concern which the World Health Organisation says is misconceived.

A BBC correspondent in Delhi describes salt as one of the most powerful symbols in Indian politics.

Gandhi's salt march challenging the colonial government's lucrative monopoly was a key event in the independence movement.