At least one Indian worker has died while cleaning sewers or septic tanks every five days since the beginning of 2017, according to the first official government statistics on the work, considered one of country’s deadliest jobs and most insidious form of caste discrimination.

But activists and the National Commission for Safai Karamacharis (NCSK), the government agency that provided the data, say the real death rate is probably much higher – with many Indian authorities still undercounting the number of such workers in their state.

Eleven Indians have died so far this month while cleaning sewers or septic tanks without adequate safety gear. Five died in a single incident last week in the west of the capital, Delhi, when they were allegedly pressured into cleaning a sewage treatment tank.

'I'm born to do this': Condemned by caste, India's sewer cleaners risk death daily Read more

“Soon after [one of the men] Umesh stepped into the plant, he fell unconscious,” a colleague of the men, Kapil Kumar, told the Hindustan Times. “Another worker entered to check on him, but he too fainted. Later, three others entered one by one, but they all fainted.”

The men were among hundreds of thousands of Indians who still work in a job known in India as “manual scavenging”, emptying dry toilets by hand, or cleaning septic tanks and sewers without protection.

Manual scavengers overwhelmingly belong to the Valmiki community, regarded among the lowest in the intricate caste system that still shapes most Indian lives.

The spate of deaths has sparked protests in Delhi and donations of more than 1m rupees (£10,480) for the family of another man who died while cleaning a sewer last Friday, after a tweet went viral showing his son crying over his body.

Manhar Valjibhai Zala, the chairman of the NCSK, said data collected so far by his agency had shown 123 deaths in sewers since the beginning of 2017. He estimated at least another 612 people had died since 1993.

The figures are the first official statistics on a problem Indian governments have sworn to eradicate since the 1950s, but NCSK officials admit they are incomplete, cobbled together based on English and Hindi newspaper articles and figures supplied by 13 of India’s 28 states and territories.

The figures also exclude those who clean dry toilets by hand, overwhelmingly Valmiki women, who do not risk asphyxiation but still develop diseases and injuries by handling faeces without protection.

Safai Karmachari Andolan, an activist group working to rehabilitate scavengers, says its data shows at least 300 people have died doing the work since the start of 2017.

Manual scavenging has persisted even as India has modernised, and is deeply embedded in the country’s caste system, transcending religion – even among Indian Muslims there are communities associated with cleaning latrines and sewers.

Members of the Valmiki community told the Guardian this year that when they migrate to Indian cities, they are excluded from any jobs but cleaning. Across Delhi, men are often seen half-submerged in sewers without protection.

Indian governments have passed several pieces of legislation outlawing manual scavenging, the latest in 2013. But many scavengers are employed through several chains of subcontractors and even when police are willing to prosecute an employer, establishing who is culpable can be unclear.

State governments are also accused of burying the problem, presenting absurdly low estimates of the number of scavengers working inside their borders. Chhattisgarh state claims to have identified only three manual scavengers in its state while Gujarat claims to have just two.

“There is no political will to solve this,” said Bezwada Wilson, the national convener of the SKA. “Enforcement agencies are not interested in eradicating scavenging because the government is not interested. If they were cases would be filed immediately and the law would be enforced.”

More protests will be held in Delhi on Wednesday demanding central and state governments crack down on the practice.