Tyson Foods has slammed a US court ruling that blocks the company from promoting its chicken as made without antibiotics thought to lead to drug resistance in humans.

A federal court in Baltimore ruled that Tyson was "misleading" US consumers by saying that its chicken was "raised without antibiotics".

The ruling came after a bid from two competitors - Sanderson Farms and Perdue Farms - to stop the ads. Both companies claim they have lost millions of dollars in sales.

The court said that Tyson was using ionophores - an approved antibiotic - in chicken feed and injecting other antibiotics into chicken eggs in the run-up to hatching.

However, Tyson said last night (22 April) that it "strongly disagreed" with the ruling and would appeal.

"Our company has complied with federal regulations throughout the development of this product line and we intend to stand our ground," said Dave Hogberg, Tyson's senior vice president of consumer products.

"Our chicken raised without antibiotics that impact antibiotic resistance in humans is more than a labelling and marketing program. It also represents a change in the way our chickens are raised, as we work to provide the kind of product nine out of ten of consumers tell us they want."

Last year, the US Department of Agriculture said Tyson could label chicken as "raised without antibiotics".

However, the agency later reversed that decision after Tyson started marketing the products. The company was eventually allowed to say its products are "raised without antibiotics that impact antibiotic resistance in humans".

Tyson said the ruling did not affect the USDA-approved label but would force the company to change its point-of-sale materials.