A genetically modified chicken named Britney, has been unveiled by the Roslin Institute in conjunction with the US biotechnology company Viragen Inc., of Plantation, Florida, which accomplished cloning of Dolly the sheep. She and her descendants are intended to join an army of special purpose medical supply animals that will be introduced to the world in the coming years.

One company, GeneWorks, of Ann Arbor, Michigan, has reportedly produced a flock of 60 such chickens, which pass on their genetic modifications to successive generations.

Each modified chicken should lay about 250 eggs per year from which a variety of proteins in relatively large volumes can be easily extracted, functioning as mini pharmaceutical plants.

One such group, Compassion in World Farming, claims that genetically modifying animals to produce medicine is unnecessary because there are alternative ways to do it.

A spokesman for Friends of the Earth, which has long campaigned against genetically modified food, said "Genetically modifying animals so they become drug factories raises serious ethical questions. The technology is well ahead of the debate."

So far, pigs, sheep, goats, bacteria and fungi have been genetically modified to produce:

• Vaccine components

Another American company, AviGenics, claims the ability to engineer its chickens to produce a chemical for treating cancer and has managed to pass on the necessary gene to further generations of birds.

AviGenics advertises itself as a specialist in avian transgenesis, and company materials say that eggs from transgenic poultry can yield large quantities of valuable proteins at relatively low cost, giving the pharmaceutical industry "a very low-cost protein expression system."

Compassion in World Farming's Joyce da Silva said: "It's as if we are determined to develop a sub-class of animals which have been tampered with so we can extract things which might possibly be of benefit to man."

Viragen Inc. is primarily known as a firm that wants to distribute worldwide a proprietary brand of interferon made from white blood cells supplied largely by The Red Cross in the United States and Europe.