New rules allowing chickens to be killed by slow suffocation in case of a widespread bird flu outbreak have been criticised by animal welfare groups.
Some 35,000 chickens were culled at a farm in Norfolk
Under a legal amendment passed on Monday, air supplies to poultry houses could be cut, leaving the birds to die.
The Department for Environment, Food and Rural Affairs stressed the method would be used as a last resort only.
But Compassion in World Farming said it could breach international welfare guidelines on minimising suffering.
Under the amendment to the Welfare of Animals (Slaughter and Killing) regulations 1995, authorised over the Bank Holiday weekend, the shutdown of ventilation system can be authorised as a method of culling in "exceptional circumstances".
Infected poultry houses could be sealed, the air supply shut off and the birds inside left to die from a combination of lack of oxygen, overheating and the effects of the disease.
Birds could take as long as a day to die, depending on their size, age and the conditions.
CIWF said ventilation shutdown was not among the disease control methods recognised by World Animal Health Organisation guidelines.
It failed to ensure the induction of unconsciousness was immediate and failed to avoid anxiety, pain, distress and suffering, it said.
Chief executive Philip Lymbery said: "Compassion in World Farming believes that death through ventilation shutdown is likely to be protracted and cause terrible suffering.
"We believe that this method is potentially so inhumane that it should not be used even as a last resort."
Defra says ventilation shut-down will be used only if no other method is possible.
Only the state veterinary service will be allowed to carry out a ventilation system shutdown and each case would have to be authorised by Defra officials.
A spokesman said: "It is not going to be an option of first instance - it's a last resort plan and an additional necessary tool to add to our options to combat this disease."
Defra's preferred culling method is rounding birds up and putting them into containers where they are gassed - the method used to cull thousands of chickens after bird flu was detected at three farms in Norfolk last week.
The spokesman said: "Ventilation system shutdown is obviously not as humane as what we did in Norfolk.
"Containerised gassing allows the birds to fall asleep within seconds and they die within minutes, so it was painless as far as they were concerned.
"But with containerised gassing, you need people and catchers to put the birds in the unit - it requires a considerable number of people."
With large farms - one of those involved in the Norfolk outbreak had 35,000 birds - this can also take a long time, he added.
In the case of a widespread outbreak, with lots of farms reporting infection, there might not be enough time or manpower to cull perhaps hundreds of thousands of birds using gas, he added.
The RSPCA says it would not endorse ventilation shutdown as a humane method of slaughter.
But head of farm animals John Avizienius said it would be "hard to conceive of a scenario where this would be needed", due to the thoroughness of Britain's bird flu contingency planning.
He added the RSPCA was concerned about how "exceptional circumstances" would be interpreted".