Ink mark applied to index fingers of every voter worn by many as symbol of democratic privilege as they visit polling boothsAre you voting in India? Share a picture of the ink on your finger

In a country where a myriad body markings, jewellery and turban styles can differentiate people by caste, region or tribe, an ink mark applied on every voter's finger at the polling booth is a rare unifying feature.

Amid crowds of Indian people heading to the polls on Thursday, the biggest day of the country's general election, the ink used to mark voters' hands to prevent fraud was flashed by many as a badge of universal democratic privilege.

The thin black mark adorns the manicured index fingers of upper class women, their hands heavy with gold bracelets and diamond rings, as well as the rougher hands of rural labourers and low-caste farmers.

The ink, which has been shipped to more than 20 countries including Sierra Leone, East Timor and Cambodia is made from a secret recipe developed by scientists in India. It cannot be washed away or erased for at least a week, sometimes up to 15 days.

But its sole global manufacturer is the legacy of a monarch whose clan ruled the princely kingdom of Mysore, unelected, for more than half a millennium.

Mysore Paints and Varnish Ltd was founded in 1937 by Maharajah Krishnaraja Wadiyar IV, one of the richest men in the world at the time. His family had owned gold mines and the Maharaja bought his favourite Rolls Royce cars in batches of seven because that was his lucky number, according to Vinay Nagaraju of Royal Mysore Walks.

"Rolls Royce dubbed his habit 'doing a Mysore'," said Nagaraju. "It means someone who spends an extravagant amount of money."

In 1962, when independent India held its third general election, scientists at the National Physical Laboratory in Delhi devised a simple ink, with small amounts of silver nitrate which reacts to light, to mark voters' hands.

"It cannot be washed away by any known chemical or solvent," said Hemanta Kumar, managing director. "When it's exposed to sunlight or even indoor light, it will darken again because of its photo-sensitive nature."

Since then, the ink, manufactured only by Mysore Paints and Varnish, has been used in every local and national Indian election, as well as in key elections across the world, including in Iraq and Afghanistan.

More than six billion fingers worldwide have been inked over the decades, according to the company.

"In some countries, the index finger is dipped in ink. Here in India, the cuticle and nail of the index finger is marked. It remains on the skin for a week, and on the nail until a new nail grows," said Kumar. This year alone, Mysore Paints and Varnish Ltd has sold 22,000 litres of ink to India's election commission, pushing its profits from £215,000 last year to £3.9 million so far in 2014, according to Kumar.

"We are very proud," he said. "We have been helping India and the world to uphold democratic credentials."