A genetically-modified chicken whose eggs contain large amounts of a protein which can be used to treat cancer could become a commercial reality.
The egg whites will contain the anti-cancer protein
Scientists at the Roslin Institute, near Edinburgh, said they had developed a way of concentrating the protein in the egg whites of designer hens.
They said they had produced a version of an antibody designed to treat malignant skin cancer.
Egg production of three other protein drugs is also being studied.
Previous attempts to manufacture drugs in the eggs of designer chickens have yielded levels too small to be of practical use.
It is hoped the new technique will allow the production of a wide range of drugs in greater volume and at a fraction of the cost of conventional manufacturing methods.
Researchers said that the first chicken egg production line could be ready within five years.
The Avian Transgenic Manufacturing project is a joint venture between the Roslin Institute, the US-based drug company Viragen Inc and the British biotech company Oxford BioMedica.
Dr Helen Sang, senior scientist at Roslin, where Dolly the Sheep - the first mammal cloned from an adult cell was created - said: "We have long believed that this joint effort would develop an avian system capable of efficiently and economically producing human biopharmaceuticals.
"With this major milestone achievement, I am even more convinced that we are developing an elite manufacturing platform that should emerge as a method of choice for many products."
Professor Alan Kingsman, Oxford BioMedica's chief executive officer, said: "Viragen is not the only company that has been trying to use chicken eggs as drug factories. But what has been achieved here is getting specific delivery of this protein into the eggs at a level that is pretty nearly commercially viable.
"This process has an efficiency of around 90%; it's way ahead of the field."
Therapeutic proteins have already been produced in the milk of genetically modified sheep, goats and cows.
But so far none of these attempts have proved commercially viable.
Other teams pinning their hopes on chickens include the US company AviGenics, of Athens, Georgia, which announced in 2003 that it had produced the human protein interferon and antibodies in eggs.
GeneWorks, of Ann Arbor, Michigan, claims to have bred more than 14,000 chickens designed to lay eggs that contain drugs.