The deadly strain of avian influenza detected in Fife was almost certainly brought to British shores by migrating water birds, and is likely to spread gradually to wild birds across the country, experts said yesterday.

Scientists and veterinarians believe migrating ducks carried the disease across the North Sea and excreted the virus into water courses when they landed.

The prospect suggests that the virus is now spreading among other wild birds in the Fife area and possibly beyond.

Officials tackling the potential outbreak immediately set up a 3km (two mile) protection zone around the coastal village of Cellardyke and stepped up surveillance for other dead birds over a 10km radius from where the carcass was found. Scientists close to the investigation told the Guardian the bird might have been dead for two weeks before it was discovered.

Extensive testing will be needed to confirm the extent of the potential outbreak, as ducks, and possibly other wild birds, can carry the infection and shed the virus without showing any outward symptoms.

Professor Albert Osterhaus, the world expert on avian flu at Erasmus University in the Netherlands, said investigations had been launched to confirm whether wild ducks were responsible for the virus reaching British shores.

"The most likely source of infection is certain species of duck, such as mallards, wigeons or pintails, migrating to the UK and excreting virus into surface waters of lakes where other birds pick it up," he said. Swans were particularly susceptible to avian flu, so acted as sentinels that the virus had reached a new area, he added.

Bob McCracken, a former president of the British Veterinary Association, said if the virus did arrive from migrating birds as suspected, other cases were almost inevitable. "An infected bird sheds a large amount of virus, so there are bound to be other birds infected," he said. "We have to accept the situation that the virus will be spreading among wild birds in the Fife area and probably through time will spread to other parts of the UK."

The area of Fife where the dead bird was discovered is regarded as important for birdlife with a high concentration of different estuarine species.

Prof Osterhaus said Britain could now expect to see a similar spread of bird flu as had been witnessed in Germany, where the virus has been transmitted slowly between wild birds, but yesterday reached a poultry farm for the first time.

Scientists were also considering the theory that the dead swan had brought the virus to Fife after leaving the Baltic more than a month ago. It was this movement of birds, largely mute swans, that led to the virus arriving in France and Germany. The virus's arrival in wild birds presents a worrying unknown for veterinarians because previous experience has charted its progress in domestic birds.

"We have no idea how this virus affects ducks, sparrows, swallows or other birds. We are having to rewrite the books on bird flu," Dr McCracken said. The incubation period of the virus is believed to be anything from two to 17 days, suggesting more cases may emerge soon.

Grahame Madge, a spokesman for the RSPB, said it would probably be impossible to ascertain whether the dead bird was local or had arrived from Europe because the species is found in both regions.

The virus has arrived as birds prepare for mass migrations from Britain to Iceland and Russia, increasing the risk of the virus being carried to those countries.

The H5N1 strain of bird flu has not been detected in Britain since 1991, when an outbreak struck a turkey farm in Norfolk. Restrictions were put place, but lifted after the outbreak was contained.