About 100 water voles are to be released in the Yorkshire Dales.
National Trust ecologists, who believe it is Britain’s largest water vole reintroduction project, are releasing the new colony into Malham Tarn, England’s highest freshwater lake.
Vole numbers have dropped by almost 90% in recent decades, becoming one of the UK’s most threatened mammals.
The animals being released have been bred in captivity and will be introduced in batches over five days.
They will spend two days in cages along the banks of the lake, before the cage doors are opened on the third day.
Ecologists will place apples and carrots on floating rafts near the cages to tempt the voles out into their new environment.
Once fully introduced into the wild, they will largely eat grass, reeds and roots.
Roisin Black, a National Trust ranger at Malham Tarn, said: “In the rest of Europe, water voles are common. In Britain, the creatures are incredibly rare.
“We know water voles have thrived at Malham Tarn in the past and thanks to work by the National Trust, the habitat here is perfect for water voles again.”
The UK’s water vole population was decimated in the 1960s, largely by American mink that had escaped from fur farms.
Water voles live in burrows dug into banks along slow-moving rivers, streams or ditches.
The population has been unable to recover largely due its natural habitat being destroyed by intensive farming, pollution and flood plains being concreted over.
Mink have not been seen in the region of Malham Tarn for 10 years.
Rangers say they will closely monitor the area for any signs of the predators by setting devices that can capture their footprints.
Ecologists hope the voles being released at Malham Tarn will improve the local ecosystem, saying their burrowing should provide the space for rare species of moss and liverwort to thrive.
They will also be food for struggling predators such as barn owls and otters.
National Trust rangers will monitor the colony over the coming year.
They should produce between two and five litters every year, with up to eight pups in each litter.
If the water voles in the reintroduction project flourish, ecologists say they plan to breed and release another 100 voles next year.