Tests are presently under way to check the remaining wildfowl on the nature reserve, where the mute swans were found.

Defra has issued a statement to confirm the H5N1 strain has been found in the birds.

Defra has stated: “Following positive test results from the Veterinary Laboratories Agency for the highly pathogenic strain of H5N1… a Wild Bird Control Area and Monitoring Area have been established around the premises, encompassing Chesil Beach and Portland Bill, and the shape of these areas is based on expert ornithological advice.”

The swans carcasses were discovered during routine Defra surveillance.

There will be, under EU law, a temporary but immediate cessation of shooting in the area of Dorset affected by avian influenza. The shooting of wild birds should cease in the protection and surveillance zones surrounding the outbreak.

Within the control zone bird owners have been told to isolate their flocks from wild birds where possible, but the culling of wild birds has been ruled out because experts fear this may possibly disperse the virus further afield.

This is the second highly pathogenic H5N1 Avian Influenza case detected in a wild bird in the British Isles. The previous case was the swan found in Cellardyke, Fife in April 2006.

Shadow Environment Secretary, Peter Ainsworth, told BBC news that: “Clearly this is very disturbing news – especially because of the connection with the wild bird population.”

The mute swans affected were not migratory birds, and normally only fly short distances.

It is highly probable the swans caught the virus from other wild birds or ducks that came into the swannery for the winter months.