Agricultural use of a last-resort antibiotic should be cut by two-thirds to limit the spread of dangerous drug resistance, European medicine regulators said on Thursday.

The demand for strict curbs on giving colistin to animals is the latest in a string of warnings about antimicrobial resistance. It follows the discovery last year of a gene that makes bacteria resistant to the drug.

The European Medicines Agency (EMA) said farm use of colistin should be limited to a maximum of 5 mg per adjusted kilogram of livestock. The drug is currently widely used in farming.

"If successfully applied at an EU level, the above threshold would result in an overall reduction of approximately 65 percent of the current sales of colistin for veterinary use," the EMA said.

Ideally, consumption should be even lower and the EMA said 1 mg or less was a "desirable" level, adding that Denmark and the Netherlands already achieved this goal. Other countries, including Spain and Italy, have far higher consumption.

Colistin, which has been used for more than 50 years in both animals and people, is given in human medicine as a last-line treatment for infections caused by multidrug-resistant bacteria, or so-called "superbugs".

Medics around the world were alarmed last year by the discovery in China of a new gene that makes bacteria highly resistant to it.

Drug-resistant superbugs are growing threat worldwide and a major review published last week called for new steps to address the problem, including limits on antibiotic use in agriculture and investment in finding new drugs.

Former Goldman Sachs chief economist Jim O'Neill, who led the review, said antimicrobial resistance could kill an extra 10 million people a year and cost up to $100 trillion by 2050 if it is not brought under control.

The exact quantity of antibiotics used in food production globally is hard to estimate, but experts believe it is at least as great as the amount used by humans.

Such routine use in farming drives drug resistance as bacteria are exposed more often to antibiotics and drug-resistant strains can then be passed to humans through the food chain.

The Health for Animals trade body, whose members include veterinary companies such as Zoetis, Bayer, Merck and Eli Lilly's Elanco, says it backs responsible farm use but antibiotics are sometimes needed. (Reporting by Ben Hirschler; Editing by Mark Potter)