English waterways could lose one of their most charismatic and once widespread residents as water voles succumb to the invasive American mink, records released by the Canal and River Trust show.

Between 1970 and 1999, water voles were found on 269 of the 2,000 miles of waterways managed by the trust. But since the turn of the century, their range dropped by almost 50% to 141 miles.

Mark Robinson, national ecologist for the Canal and River Trust, said the numbers told of a species in desperate decline.

“I very rarely hear good news about water voles. Whether extinction [from England] will occur or whether we will turn the tide I don’t know, but I think it’s certainly in the balance,” he said.

The introduction of the American mink, which have escaped from fur farms in Britain since the 1920s, has been the single greatest driver of decline. Mink are voracious predators and will hunt voles.

“That has driven whole river systems to a complete population crash,” said Darren Tansley, a wildlife officer with Essex Wildlife Trust. “The possibility [that they will become extinct in England] is there if nothing is done to control mink.”

The development of river banks and water pollution has added to the pressure on one of Britain’s most endangered mammals. The industrialisation of river systems during the 1970s and 1980s destroyed the banks where voles make their burrows.

Tansley said although the Canal and River Trust’s numbers were based on ad hoc observations not scientific surveys, they were consistent with the observed decline in all English waterways. Water vole populations are very difficult to estimate, but Tansley said the natural population of tens of millions had now almost certainly dropped below one million.

All counties have suffered “dramatic declines”. A survey in Essex in 2006 found most main rivers “utterly devoid of water voles”. The animal is now extinct in Cornwall. Across the country water voles cling on in isolated pockets, coastal marshes and backwaters where the mink has not found its way. In Scotland water voles have fared better by behaving differently to their English cousins, often living on land and thus avoiding the mink invaders.

Invasive American minks first escaped from Britain’s fur farms in the 1920s, since then they have been the main predators of water voles. Photograph: Stephan Morris/Alamy

Where conservation programmes to control the mink have been put in place, Tansley said the vole population had rebounded. Unfortunately, he said, mink do not pose an economic threat and therefore controlling its spread was not seen as a priority and attracts no funding from the government. The fight to save water voles has been almost entirely led by charities.

The trust has called for the public’s help in a mass citizen science survey of the species – the Great Nature Watch. Robinson asked people to download the eNatureWatch app and log their vole sightings.

“They are just part of our rich wildlife, right back to Wind in the Willows, that we need to look after and protect. And the more we know about where they are, the better we can protect them,” he said.

In a subplot worthy of a sequel to Wind in the Willows (which featured a water vole named Ratty), some wildlife experts have suggested the resurgent otter may prove an unexpected ally for the vole by ousting the weasel-like mink.

“There’s anecdotal evidence that the comeback of otters is reducing the impact of mink by driving them out. I think they will contribute [to the survival of the vole]. I don’t think they can do the job on their own,” said Robinson.

But the idea divides experts. Tansley said otters would certainly prey on mink, but there were many instances where the two species inhabited the same river system.

“I am afraid I am not a big proponent of the idea that otters push mink out ... since they coexist with otters elsewhere in Europe and with an ecologically equivalent otter species in their native US,” said Dr Lauren Harrington, a wildlife expert at the Zoological Society of London.