As India heads to the polls starting next month, workers hunched over boxes at a factory in the southern city of Mysuru race to finish producing millions of bottles of indelible ink for the world's largest election.

For decades, India has used the ink, made primarily from silver nitrate, to mark voters after they have cast their ballot to prevent duplicate votes and fraud.

Founded in 1937, Mysore Paints And Varnish Limited is the only company authorised to produce the ink in India. When exposed to sunlight, the ink stains the skin and fingernails purple, lasts for about two weeks and is almost impossible to erase.

Since the start of the year, the company has shipped a record 2.7 million ink vials that will be used to mark the nearly 970 million people registered to vote.

The company is manufacturing 80,000 bottles daily to meet the production target for the election, according to quality control manager Vishalakshi K.

India's states independently engage with the company for their needs. The most populous state of Uttar Pradesh placed the biggest order, while the tiny island territory of Lakshadweep put in the smallest order for just 110 vials, company executives said.

Each vial costs 174 rupees ($2.10), which managing director Mohammed Irfan said was a rate determined by India's election commission, and can be used to mark about 700 voters.

This will give the company, whose main business is making paints used on public transport vehicles, an income in excess of $7 million from the vote.

Mysore Paints also supplies the indelible ink to other countries and counts several in Asia, such as Malaysia, Maldives and Cambodia, among its customers.

India will elect a new parliament in seven phases between April 19 and early June.

Opinion polls have predicted an easy win for Prime Minister Narendra Modi, which would make the 73-year-old only the second prime minister to win a third straight term. - Report from Reuters