The number of hazel dormice in Britain is said to have halved since 2000.
Numbers are down 51% and they have vanished from 17 English counties, according to data gathered by volunteers for the National Dormouse Monitoring Programme.
They are now almost entirely found south of a line between Shropshire and Suffolk.
The hazel dormouse is the only one native to Britain.
The tiny mammals have been affected by the way woodlands are managed, said the People’s Trust for Endangered Species (PTES).
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Its State Of Britain’s Dormice report says it is vital to provide the right habitat to bring them back from the brink.
They like mixed woodland: tree holes to nest in, dense shrubby areas under trees, and hedgerows to move around the countryside.
But a change away from traditional woodland management has made it difficult for them.
Extreme or unseasonable weather can also affect their chances of making it through the winter and their ability to breed.
It’s not all bad news – populations were found to be stable or increasing at 96 of the 336 sites looked at.
Ian White, dormouse and training officer at PTES, said: “The decline in dormouse numbers is due to the loss and fragmentation of their natural woodland and hedgerow habitats, as well as climate change.
“In particular, it’s the loss of habitat quality that’s of real concern.”
He added: “We can help bring this species back if we alter the way we manage our landscape.
“By providing enough of the right habitat, which is well-connected and managed correctly, dormice, as well as a huge amount of other wildlife, can thrive once again across the country.”
The PTES has carried out 30 reintroductions at 24 sites over the past quarter of a century, releasing almost 1,000 dormice to create new populations or boost numbers.
It has also just launched the Great British Hedgerow Survey, asking farmers to assess the condition of their hedgerows, which help connect habitat areas for dormice.
