The US Food and Drug Administration has told four dairy products firms to stop labelling their products as being free of hormones.

The FDA said no milk product could be labelled hormone free because all milk contained naturally occurring hormones.

"FDA is committed to assuring that consumers are provided with truthful information on product labels," FDA Commissioner Dr. Mark McClellan was quoted by Reuters as saying.  

The agency said the four companies, which produce milk and ice cream, may have been aiming to say that their products were free of an engineered hormone called recombinant bovine somatotropin or rbST, which is used to increase milk production.

"Before the 1993 approval of rbST, FDA determined that the recombinant, or genetically engineered form of bST is virtually identical to a cow's natural somatotropin, a hormone produced in the pituitary gland that stimulates the production of milk," the agency said.  

The FDA said that because the products were virtually identical there was no basis for claiming that milk from non rBST-treated cows was safer than milk from cows treated with the engineered hormone.