The UK's water vole population has decreased by 22 per cent since 2011, reports the Environment Agency and The Wildlife Trusts.
Numbers of the semi-aquatic rodents - immortalised as Ratty in Wind in the Willows - have declined by a fifth, with animals particularly vulnerable in the South West, the South East, the Lake District and parts of the Midlands.
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The depletion is down to habitat loss, extreme weather events like last spring's drought and an increase in the presence of vole predator the American mink.
Paul Wilkinson, The Wildlife Trusts' head of Living Landscapes has called for more action to save the mammals.
He says: 'There is clear evidence from some areas, in the south of England for example, that water voles are disappearing fast.
'Not enough is being done to secure this charismatic species' future.'
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The animals flourish in areas with extensive wetlands and some strongholds remain where living conditions are more vole-friendly.
It's hoped that by creating and maintaining more large areas of suitable habitat - with decreased mink populations - the mammals will be able to thrive again.
The Environment Agency's National Conservation manager Alastair Driver explains: 'The Environment Agency has created nearly 5,000 hectares of wetland and river habitats in the last 10 years and we hope to double this in the next 10.'
* Find out why water voles are one of our favourite animals
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