British scientists have developed genetically modified (GM) chickens that cannot transmit bird flu infections -- a step that in future could reduce the risk of avian flu spreading and causing deadly epidemics in humans.

Scientists from Cambridge and Edinburgh universities said that while the transgenic chickens still got sick and died when they were exposed to H5N1 bird flu, they didn't transmit the virus to other chickens they came into contact with.

"Preventing virus transmission in chickens should reduce the economic impact of the disease and reduce the risk posed to people," said Laurence Tiley, of Cambridge's department of veterinary medicine, one of the lead researchers on the study.

H5N1 bird flu has been circulating in Asia and the Middle East, with occasional outbreaks in Europe, since 2003 and has killed or forced the destruction of hundreds of millions of birds, according to the world animal health organization OIE.

It rarely infects people but when it does it is deadly: the World Health Organization has documented 516 cases in people since 2003 and the virus has killed 306 of them.

Experts say the danger is that the virus will evolve into a form that people can easily catch and pass to one another, causing the transmission rate to soar and producing a pandemic in which millions of people could die.