The dead swan that tested positive for avian flu after washing up on the Scottish coast may have contracted the disease in another country and died as it attempted to migrate across the North Sea.

Officials investigating the discovery of the lethal H5N1 strain of bird flu in Britain yesterday revealed a "working hypothesis" that the bird had already succumbed to the virus before washing up in Cellardyke harbour.

The announcement came as DNA tests showed the bird was not a mute swan, as originally believed, but a whooper swan, a species that winters in Europe before migrating to Britain.

Experts welcomed the news that the bird was not native, as it suggests Britain's case of bird flu might be a one-off, easing fears that the virus is circulating undetected among wild birds. No birds have tested positive for the virus since the dead swan was recovered.

Tests on the virus that killed the swan indicate that it is almost identical to a strain that has infected more than 100 birds in Germany.

"It looks like this is a whooper swan that spent the winter in Europe, set off on its migration, got halfway across the North Sea, felt like crap, and landed and died before washing up in Cellardyke." said Andre Farrar, a spokesman for the Royal Society for the Protection of Birds. "It can put the spring back in people's step because it makes it less likely that other positives will be found."

He added: "What supports the theory is that unlike mute swans, you don't see whooper swans bobbing around in harbours, they head to freshwater wetlands."