Chickens raised in India for food have been dosed with one of the strongest antibiotics known, a study by non-profit news organisation The Bureau of Investigative Journalism said on Tuesday.

Citing the example of a farm in Rangareddy district near Hyderabad, the bureau said chickens are fed colistin, which doctors call a “last hope” antibiotic because it is used to treat patients who are critically ill with infections resistant to most drugs.

Antibiotics help the chickens remain free of disease or gain weight faster. But the World Health Organization, which calls antibiotics like colistin “critically important to human medicine”, restricts their use in animals and bans them as growth promoters, the study said.

The indiscriminate use of colistin in farming increases the chance that bacteria would develop resistance to the drug, making it useless when treating patients, the study said.

The bureau said that thousands of tonnes of colistin has been shipped to Vietnam, India, South Korea and Russia in 2016, for veterinary use. In India, at least five pharmaceutical companies openly advertise products containing colistin as a growth promoter. The drug is manufactured by two companies in India, but the country also imports 150 tonnes of it every year.

One of the companies which advertises colistin in India is Venky’s. The company also supplies meat to fast food chains like KFC and McDonald’s. The bureau said it had bought Venky’s-branded colistin from a poultry feed and medicines shop in Bengaluru without a prescription.

However, the company told the bureau that it was not breaking any laws by selling colistin in India. “Our antibiotic products are for therapeutic use – although some of these in mild doses can be used at a preventive level, which in turn may act as growth promoters,” the company said. McDonalds, KFC, Pizza Hut and Domino’s claimed that the chicken they sourced from Venky’s was not raised on such antibiotics.

Drug-resistant bacteria kill 7,00,000 people worldwide each year, the study said. The toll may increase to one crore a year by 2050 if no action is taken, with 47 lakh deaths occuring in Asia.

“Colistin-resistant bacteria will spread on the chicken farms, in the air surrounding them, contaminate the meat, spread to the farm workers, and through their faeces flies will spread it over large distances,” Timothy Walsh, a global expert on antibiotic resistance, told the bureau. He said the drug should only be used on very sick patients and not exported for feeding chickens.

England’s Chief Medical Officer Dame Sally Davier called for worldwide ban on colistin and other antibiotics as growth promoters.