There may be only 50 slender-billed curlews in the world
The slender-billed curlew, the rarest bird to be seen in this country since the extinction of the great auk 200 years ago, has been accepted on to the official British list by the British Ornithologists' Union Records Committee.
With a global population possibly as low as 50, the sighting has given hope for the species' continued survival.
Birdwatcher Tim Cleeves found and identified the curlew at Druridge Bay in the spring of 1998.
On Thursday, it formally accepted the 1998 record - the last time this species has been seen anywhere in the world.
Mr Cleeves, a RSPB conservation officer based in Yorkshire, spotted the bird among a flock of curlew, its larger and widespread European cousin
He said: "Although there have been claimed sightings of other slender-billed curlews since 1998, none of these have been officially accepted.
"I could have the dubious honour of being the first birdwatcher in Britain to see a slender-billed curlew in Britain and one of the last in the world to see one alive."
The sighting has triggered a high-tech RSPB- led research project, to pinpoint the breeding grounds of this enigmatic bird.
The slender-billed curlew is so rare that only one nest has ever been discovered and that was more than a century ago.
Despite many subsequent expeditions, the bird's main breeding grounds, thought to lie in the wildernesses of western Siberia or northern Kazakhstan, have eluded detection.
After studying the Northumberland bird's plumage on hours of videotape and a number of photographs, experts decided the bird was less than one year old.
It proves that at least in 1997, it was still nesting successfully somewhere on the planet.
The society's researchers will now carry out further tests from feathers of slender-billed curlews held in museums.
Any feathers the bird had grown at a particular breeding ground would reflect the area's "signature" in their isotope ratios (the relationship between different types of the same atom), narrowing the search area for researchers.
Dr Debbie Pain, an RSPB research scientist, said: "The fens of western Siberia are being drained rapidly and climate change and desertification appear to be affecting the steppe grasslands of Kazakhstan and southern Russia.
"If we are to save this bird we need to find its breeding grounds as a first step and work to ensure their adequate protection."