Beavers were made extinct in England hundreds of years ago. But at the River Otter, which goes through Somerset and Devon, they were re-introduced to the wild from captivity more recently.
The government originally said the beavers needed to be captured because of disease, but said they should be allowed to stay there permanently, saying they were protected under EU law and previously made extinct by humans.
Authorities have now allowed the beavers in River Otter to stay for a five-year trial period, as long as tests show they aren't carrying disease and the population is controlled.
Andy Lester, from A Rocha, told Premier: "This is a fantastic result for local wildlife enthusiasts and actually for those who are concerned about the river's future.
"They used to be wild in the UK a hundred years, but were hunted to extinction for their fur. So having beavers back in Devon is going to be a great asset.
"There's very little concern. This is going to be a controlled population so we're not talking about allowing beavers to breed all the way down the river in very large numbers, the population's likely to be carefully monitored and controlled.
"The big advantage of beavers is that they can slow river flow down. And that of course is good news for those who are concerned about flooding.
"So it's likely to reduce the likelihood of flooding locally."
Andy Lester, A Rocha: