The stakes were raised last month in the battle to determine whether food from cloned animals and their progeny should be allowed on the shelves of US grocery stores.

In December, Congress urged the Food and Drug Administration (FDA) to keep in place a request that companies voluntarily refrain from selling such foods. It also said that the US Department of Agriculture should study the economic implications of allowing meat and milk from cloned animals into the food supply.

The FDA had been expected to finalize in 2008 a preliminary assessment it made just over a year ago, which concluded that foods from cloned animals and their progeny are not different from products from conventional animals and could probably be sold without special labelling.

On 19 December, the two leading US animal-cloning companies (ViaGen in Austin, Texas, and TransOva Genetics in Sioux Center, Iowa), announced a radio-tracking programme that will use ear tags to follow cloned animals from birth to death. The firms said that the voluntary programme will come into effect as soon as the FDA allows food from clones on the market.