The water vole colony was destroyed by non-native mink
Around 200 water voles are to be reintroduced at a lake where its population was destroyed after mink were released there.
The American mink, a member of the weasel family, were originally brought to Britain for fur farming in 1929.
But they have been a major factor in the decline of the native water vole.
The Environment Agency will reintroduce the vole back to Llangorse lake in Brecon, Powys after mink were cleared from the site.
They had intended to carry out the release on Monday, but were delayed because they did not have enough boxes to transport the animals to the site.
The release will now take place on Monday 8 June, a spokesman for the Environment Agency said.
Water voles could traditionally be found at the lake but a release of mink destroyed the native population.
And after work to trap the mink and restoration of the vole's habitat, the agency is planning to release 200 of them to the area.
The water voles were bred in captivity at a specialist hatchery run by the agency.
The American mink is non-native to Britain and all wild mink of its type found in the British countryside are descendants of escapees.
Their presence has been detrimental to creatures like the water vole because they are small enough to follow their prey down its burrow.
Mink also feed on birds and their young when they are nesting.
As part of its work to protect Britain's native species, the agency is also planning to trap mink which are living at a site in Deeside, Flintshire, which is threatening Wales' largest colony of terns.
In 2008, 60-70 fledglings were taken from nests of the 400-strong colony by the mink.
The agency plans to find and trap mink living at the site in order to protect this year's chicks.