Three other wild swans tested positive for the virus last week after they were found dead at the Abbotsbury Swannery in Dorset, an open reserve in the Chesil Beach area, during routine surveillance.

A fourth dead swan was confirmed to have tested positive on Thursday.

John Houston, general manager at Abbotsbury Tourism Ltd, said: "One more swan has tested positive.

"We are expecting to have a run of positives and negatives while it works its way through the herd."

Restrictions on poultry movements in the area were imposed after the dead birds were confirmed to be infected with the strain of bird flu last Thursday.

The Environment Department said there was currently no evidence to suggest the disease was widespread among wild birds in the area, but enhanced surveillance was taking place.

The Abbotsbury Swannery will be given the all-clear after 21 days with no birds testing positive for the virus.