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 human epidemics.

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.

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 Organisation for Animal Health, OIE.

It rarely infects people but when it does it is deadly: the World Health Organisation 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.

To breed their GM chickens, the researchers introduced a new gene into them that manufactures a small "decoy" molecule that mimics an important control element of the bird flu virus.

The replication machinery of the virus is tricked into recognising the decoy molecule instead of the viral genes and this interferes with the virus' replication cycle.

After producing the modified chickens, they infected ten of them and ten normal chickens with H5N1 bird flu. Like the normal chickens, the transgenic birds became sick with the virus, but they did not transmit the infection on to other chickens kept in the same pen with them.