Protesters in Gloucestershire campaigning against badger culling have condemned Defra after it admitted it had been undertaking gassing trials.
An independent expert panel concluded pilot badger culls in Gloucestershire and Somerset were inhumane and ineffective.
Shooting and cage trapping methods failed to kill the target 70% of the population in both areas to reduce bovine tuberculosis.
Mark Jones, executive director of Humane Society International UK, said he was disgusted with the trials and called for a halt immediately.
Mr Jones said: “I am appalled Defra has secretly been spending public money testing gassing as a potential method of killing badgers.
“It shows a chilling and arrogant determination by this Government to massacre these wild mammals at all costs.
“Defra’s shambolic badger cull last year showed free shooting is inhumane and won’t work and that cage trapping is hugely expensive, so now it is looking at gassing these protected wild mammals in their setts.
“Previous gassing studies saw cats, dogs, guinea pigs and monkeys enduring anything from tremors and vomiting to severe lung pain, heart attack, suffocation, coma and death.”
A spokesman from Defra said it was conducting “initial investigations”.
He said: “The investigations do not involve animals and it is not possible to say at this very early stage if or when gassing is likely to be a realistic or humane method of culling.
The spokesman added: “There will be some preliminary tests to investigate the dynamics of carbon monoxide dispersal in a sett environment to determine whether any available delivery mechanisms have the potential to achieve humane and effective outcomes in real sett situations. This will not involve the use of active setts or tests on live animals.”