An emergency breeding programme is being set up to save the species, whose known population is down to the one lone male. English Nature, the Government's wildlife agency, is joining with Anglian Water to breed the animal with other pool frogs from Scandinavia, which are closely related. It is hoped that this will eventually allow a reintroduction programme to take place in the Fens of East Anglia.
Found over much of Europe, the pool frog, Rana lessonae, is most readily distinguished from Britain's common frog, Rana temporaria, by its noise - it blows out its cheeks to make a sound louder than a quacking duck. The common frog has non-inflatable cheeks and makes little noise, even if provoked. The pool frog also has a cream stripe down the length of its brown back, and enjoys basking in the sun, while common frogs have no stripe and prefer to hide in the grass.
It has been known in two sites in England - Cambridgeshire and Norfolk - but until recently it was thought to be a 19th-century introduction from the Continent, like the edible frog Rana esculenta which can also be found in some parts of the South, and was largely ignored by naturalists.
Research in the Nineties, however, has revealed that East Anglia's tiny population of pool frogs are very likely native - they have been there since time immemorial - and are thus our rarest vertebrate animals. But their population has been devastated by the constant draining of the Fens since the 17th century.
Now Anglian Water is providing pounds 30,000 to help English Nature confirm their uniqueness with studies of their DNA, a further survey to establish if any remain in the wild and a breeding and reintroduction programme. "Without urgent action the loss of this animal is inevitable," said Stephen Bolt, Anglian Water's head of environmental affairs.
Having died out in Cambridgeshire in the 1880s, the pool frog's last remaining known site was a series of Norfolk ponds on a common near Thetford. Enthusiasts concerned for the animal's future collected three there in 1992 to try and breed them. All were males, so two were released and one was kept. But there has been no sign of any further frogs in the wild.
The surviving male - they can live to be nearly 20 - is being looked after by a biology teacher and amphibian enthusiast, Charles Snell, who in a 1994 paper in the journal British Wildlife first made the case for the pool frog being a native British species. He keeps the frog in a large enclosure in a suburban garden whose location he is not keen to disclose in case of theft by other frog breeders. "There are some fanatics out there," he said.
Next spring he hopes it will breed with female pool frogs from Sweden, which are genetically quite close to the British species, to enable a reintroduction programme to begin. "The Norwegian pool frogs are even closer, but they are very rare," he said.
Mr Snell is now carrying out some of the DNA research in the laboratories of Greenwich University. "This is an overlooked species which is probably our rarest native vertebrate," he said.
Anglian Water has three sites earmarked for possible reintroduction of the frog.
BRITAIN IS not blessed with amphibians - frogs, toads and newts, those animals related to fish which live largely on land but need water to breed.
Compared to the dozens of species found across the Channel, we have hitherto had only six regarded as native.
There are three newts - the common newt, the palmate newt and the rare and strictly protected great crested newt. There are just two toads, the common toad and the rare and protected natterjack toad, and a single frog, the common frog. The pool frog will be the seventh native British amphibian - if it can successfully be reintroduced.