A disease epidemic is killing millions of Britain's most common and least studied amphibian, the frog - and the cause could be goldfish imported from the US.

Soon frogs should be engaged in the annual breeding frenzy which leads to the piles of frogspawn in garden ponds - to the delight of children and of gardeners, who rely on frogs to keep down the slug population.

But large areas of London and the south-east have seen a wiping out of the frog population on a scale reminiscent of myxomatosis in rabbits. The disease causes sores and sometimes the loss of whole legs as the frogs die slowly.

So far there have been 3,500 documented cases of the disease, with a body count of 62,000, the worst outbreak killing 2,000 frogs in a single incident.

Tom Langton, director of the Froglife Trust, who has been studying the spread of the disease, said it was killing populations south of a line between Dorset and Norfolk, but cases had been found in the Midlands, Wales and Scotland.

The disease was first noted in the early 90s, spread rapidly, and began to threaten whole populations. He said: "They may be wild animals but people get attached to them and they are almost in tears when they ring to ask what to do. The frogs look so desperate with sores and toes falling off as they get weaker and weaker."

Mr Langton, with Andrew Cunningham and Peter Bennett of the Institute of Zoology at Regents Park, the research arm of London Zoo, has been looking for a cause of the epidemic.

A virus known to be able to jump species has been identified as the cause. A ranavirus found in north America is the main culprit. This appears to have arrived in Britain from goldfish farmed in the US and brought to Britain to put in garden ponds. Goldfish and frogs both dying from ranavirus have been found sharing the same ponds.

The virus also affects the north American bullfrog, brought to this country as tadpoles to be sold in pet shops in the 80s. Another theory is that Americans with shotguns, shooting bullfrogs in fish farms, blasted the pieces into the ponds, which were eaten by the fish, which were subsequently imported to Britain bringing the infection with them.

Since the virus also attacks other reptiles, and a pet London tortoise has died of it, there are fears that other British species may also suffer population crashes.

A complicating factor in the study is the presence of large quantities of copper in the livers of frogs, which would damage their immune system and make them more susceptible to disease. Many of the dead frogs were recorded as having higher concentrations of copper than previously known to science.

The main culprit for this is slug pellets, which include copper to give them their characteristic blue colour designed to make them unattractive to birds. Frogs feasting on half dead slugs, some with slug pellets attached, could be destroying their own immune systems allowing them to fall victim to the virus.

Mr Langton said: "We have a fantastic amount of information on this disease and not the financial means to continue studying it. Foot and mouth cost £3bn and BSE even more, what we need is £1m to study the common frog and find out what is wiping it out."

· The frog is Britain's most common amphibian, although no one has ever counted them

· Frogs hop up to 2ft, have webbed feet, and wet looking skin, unlike toads which mostly crawl, have claws and are warty

· They can be green, red, brown, grey or yellow

· Frogs grow all their lives, reaching 10cm (4in) in six or seven years, but most do not last that long, being eaten by herons, foxes, hedgehogs, domestic cats or grass snakes, or run over

· Frogs breed in still water, mostly in garden ponds. The young emerge in wet weather in mid-summer and return to breed two or three years later

· Most frogs are still hibernating, burying themselves in damp ground or in mud at the bottom of ponds

· Food includes slugs, worms and flies and many insects