Tests were continuing last night on birds found close to the reserve where three wild swans have tested positive for the deadly H5N1 strain of bird flu.
Restrictions on the movement of captive birds are to remain in place following the discovery of the dead swans at the Abbotsbury Swannery, an open reserve in the Chesil Beach area of Dorset, during routine surveillance.
Routine tests were being carried out by the Department for Environment, Food and Rural Affairs (Defra) on two birds found at or near the reserve.
A Defra spokesman said: "As soon as results are available, they will be published."
One of the two birds was found in the same nesting area at Abbotsbury where the first three swans were discovered, while the second was found about three miles away. It is possible that it hit power lines, the swannery said. Two other dead swans were also found miles apart at opposite ends of a lagoon, John Houston, the general manager at Abbotsbury Tourism, said. This meant it was "less likely" that the birds had infected each other.
Mr Houston added: "It's not unusual for birds to die in the winter of natural causes. In fact, there are fewer dying at the moment than normal because it's quite warm." An average of 20 to 30 swans usually died in January.
David Wheeler, a swan herder, said: "There's no reason to worry. But having said that, we could lose one or two more, you can't predict. We know swans can be susceptible."
The Government's acting chief veterinary officer, Fred Landeg, said: "Our message to all bird-keepers, particularly those in the area, is that they must be vigilant, report any signs of disease immediately, and practise the highest levels of biosecurity."
The public can help the effort by reporting any incidents, such as large numbers of sick birds, to the helpline on 08459 335577.