A case of deadly H5N1 bird flu found in wild swans in Dorset earlier this month was probably introduced by an infected migratory bird, Defra has said.
The virus is not thought to have spread to wild birds in the area
Six swans from the Abbotsbury Swannery have tested positive for the disease.
An epidemiology report said the strain of the virus is similar to that found in Europe in the latter part of 2007.
The Defra report said it is not possible to conclusively identify the source, but a migratory bird is the "most likely hypothesis".
There is no evidence of widespread infection in the wild bird population in the area and no evidence of disease in domestic birds, the report said.
But Defra has urged local poultry keepers to remain vigilant and report any signs of disease immediately.
The first birds to test positive were found dead at the open reserve in the Chesil Beach area in December during routine surveillance, while further swans were found to have the disease earlier this month.
Restrictions on the movement of poultry and other captive birds in the wild bird control area, imposed in the wake of the first positive tests, remain in place, according to a Defra spokeswoman.
However, movement restrictions in the wider wild bird monitoring area around the Abbotsbury Swannery were lifted last week.
She said there is ongoing surveillance of wild birds in the area, although the disease so far appears to be confined to the six swans - representing a very low level of infection in the wild bird population on the site.
The swannery is part of an area of wetland recognised to be of international importance.
About 12 members of staff at the sanctuary have been monitored for signs of the disease by the Health Protection Agency, although the risk of infection is said to be low.
Workers have been given a course of the anti-viral drug Tamiflu as a precaution.