A wildlife hospital says it is being forced to destroy injured grey squirrels.
A change in the law means animal rescue centres are no longer allowed to return non-native species back to the wild after treatment.
Leicestershire Wildlife Hospital says the legislation means it has no choice but to kill the squirrels humanely because it has no room to keep them.
The hospital in Kibworth told the BBC the new law effectively forces staff to euthanise the animals.
The law around releasing invasive non-native animals came into effect last October.
Grey squirrels are vulnerable because they were introduced to the UK in the 1800s from North America.
They have been blamed for the decline in Britain’s red squirrel population.
The government says non-native species are a threat to the UK’s native biodiversity.
The new law also affects muntjac deer, which were introduced to Britain from China in the 20th century.
The Wildlife and Countryside Act 1981 already made it an offence to release grey squirrels after treatment without a licence, but rescue centres could apply for one from Natural England.
But the new EU law, the Invasive Alien Species (Enforcement and Permitting) Order, prevents this.
Rescue centres could face prosecution for releasing a grey squirrel back into the wild.
Leicestershire Wildlife Hospital has told members of the public not to bring in injured squirrels.
Harriet Childs, a team leader at the hospital, told the BBC: "We're having to euthanise them for just being what they are and it's not fair.
"We had to do one on Sunday and it's a bit raw still.”
She said the hospital takes in more than 5,000 injured animals each year.
She said if they do take in grey squirrels or muntjac deer then “we have to euthanise them or keep them in a cage for the rest of their life”.
The RSPCA told the BBC it was “extremely disappointed” that licences to release grey squirrels are not being reissued.
It said the legislation will “result in the needless destruction of animals”.
The Department for Environment, Food and Rural Affairs said the legislation is necessary to restrict the impact of invasive non-native species.