Factory farming and the international poultry trade are largely responsible for the spread of bird flu, and wild birds are being unfairly blamed for the disease, a new report says.

The report says the deadly H5N1 virus developed inside intensive poultry units in Asia and has proliferated through exports of live birds and the use of chicken droppings as fertiliser. Its publication by Grain, an agricultural pressure group, follows an announcement that the virus has been found in a turkey farm in eastern France. Though the farm was close to where two infected wild ducks were found, all its 11,000 turkeys were kept indoors with no contact with wild birds.

Dissident scientists accept that the flu began in wild birds, but say it developed in the cramped conditions of Asian factory farms. Research published in the official journal of the US National Academy of Sciences blames the poultry trade for the virus spreading from China to Vietnam.

BirdLife, a charity, says the virus"s spread across Russia last summer - widely attributed to migrating birds - took place when birds were moulting and unable to fly. It adds that an outbreak in Nigeria took place on a factory farm far from migratory routes.