A flock of designer hens, genetically modified so that they lay eggs with drugs to fight cancer and other life-threatening diseases, has been created with the help of Oxford scientists.

Researchers from Oxford Biomedica helped colleagues at the Roslin Institute near Edinburgh, which created Dolly the cloned sheep, to breed a 500-strong flock of transgenic' hens, to deliver eggs containing drug therapies at a fraction of normal cost.

The proteins in the whites of the eggs could be used to cheaply mass-produce drugs which cost several thousand pounds a year for each patient.

Oxford Biomedica chief executive Alan Kingsman said: "We do gene therapy in people, developing therapies for cancer and other diseases.

"Another company, Viragen, asked to use our gene transfer technique for this because we have developed some of the most powerful methods of using engineered viruses in cells."

Conventional methods for producing protein drugs are expensive and time-consuming, although scientists have successfully made a range of these molecules in the milk of genetically modified sheep, goats, cows and rabbits.

The GM chickens are reported in the Proceedings of the National Academy of Sciences by Dr Helen Sang and colleagues in the Roslin Institute, Oxford Biomedica, based at Oxford Science Park, and Viragen, which is commercialising the technology.

They describe how they have produced transgenic hens by using Biomedica's horse virus to insert genes for pharmaceutical proteins into the gene which allows chickens to produce ovalbumin, the protein that makes up 54 per cent of egg whites.

Prof Kingsman said the Oxford technique disabled the horse virus so that only ten per cent remained with no risk of disease.

Professor Harry Griffin, director of the Roslin Institute, said: "The idea of producing the proteins involved in treatments in flocks of laying hens means they can be made in bulk and cheaply.

"Indeed the raw material for this production system is quite literally chicken feed."

Roslin has bred about 500 modified birds, but it could be another five years before patient trials get the go-ahead and 10 years until a medicine is fully developed, the Roslin Institute said.