"We don't see vaccination as 100 percent effective," a Department for Environment, Food and Rural Affairs (Defra) spokeswoman said on Wednesday.

"You have to vaccinate every single bird and even then a vaccinated bird can catch the virus and become a carrier without showing any symptoms."

Six countries have banned British poultry imports after the outbreak was confirmed on a turkey farm in Suffolk on Saturday.

The decision puts the onus back on farmers, some of whom want to vaccinate their birds but fear that unless vaccination is widespread, they will be at a competitive disadvantage because some export markets shun vaccinated poultry.

"I think that many organic and free-range producers who are committed to their birds would have their birds vaccinated," said Richard Sanders, head of Policy at Elm Farm Research Centre, a leading organic farming research and advisory body.

Source: Reuters