Antibiotics crucial to human medicine are still being used in “unacceptable” quantities on US livestock farms, despite rules brought in last year intended to curb their use and combat the spread of deadly superbugs.

Tests on thousands of meat samples carried out by the US Food Safety Inspection Service (FSIS) show that farm animals are still being dosed with powerful antibiotics classified as “critically important” to human health. The widespread use of such drugs on livestock is one of the key drivers of antibiotic resistance, a growing public health crisis.

Regulations were brought in by the Food and Drug Administration (FDA) in January 2017 which banned the use of antibiotics on livestock without a prescription from a vet and made it illegal to use the drugs solely to make animals fatter, which for years had been a common practice on industrial farms.

The new rules aimed to curtail the overuse of antibiotics and make sure they were only used when medically necessary. However the tests on livestock slaughtered at dozens of US meat packing plants – including some operated by major processors such as Tyson, Cargill and JBS - found “critical” antibiotics still in use in many meat supply chains. There had been no reduction in the number of antibiotics found in samples from the year before the regulations came into effect.

Analysis by the Bureau also shows how a loophole means US farmers can still use many antibiotics targeted by the ban in much the same way today as they could beforehand, including drugs previously used for growth promotion.