LONDON (AFP) - Three swans found dead on a nature reserve in south-west England have tested positive for the H5N1 strain of bird flu, the Department for Environment, Food and Rural Affairs said Thursday.

Defra said avian influenza was confirmed in the wild mute swans near Chesil Beach, Dorset "following positive test results for the highly pathogenic strain of H5N1."

"While this is obviously unwelcome news, we have always said that Britain is at a constant low level of risk of introduction of Avian Influenza," said acting chief veterinary officer Fred Landeg.

"Our message to all bird keepers, particularly those in the area, is that they must be vigilant, report any signs of disease immediately, and practice the highest levels of biosecurity."

Media reports said urgent tests were under way to determine whether the virus -- which can pass to humans -- has infected other birds and ducks at a bird sanctuary, in Abbotsbury, Dorset.

John Houston, in charge of Abbotsbury Swannery, told BBC News 24 television: "We are working closely with Defra (Department for Environment, Food and Rural Affairs) to ensure that this outbreak is contained and that the swans affected is low.

"We are also working with the Health Protection Agency to make sure that the public and staff are protected."

The swannery, which was originally set up in the 11th century by monks who regarded swan meat as a delicacy, is a popular visitor attraction, but closed on October 28. It is due to reopen on March 15, Houston said.

About 800 swans, as well as other birds, inhabit the wetlands, which are close to the south Dorset shore, to feed and breed.