Emergency legislation was placed before Parliament during the May Day Bank Holiday giving Margaret Beckett, the Rural Affairs Secretary, the power to order “ventilation shutdown” at chicken farms. This would remove oxygen flow from chicken houses.

Birds could take up to a day to die, depending on their age and size and the time of year. Death would be caused by a combination of overheating, bird flu and lack of oxygen.

The plan has outraged animal welfare organisations, which are demanding clarification about the method of slaughter. Compassion in World Farming (CIWF) and the RSPCA said that the powers were in breach of standards laid down by the World Organisation for Animal Health, based in Paris. They are demanding to know under what “exceptional circumstances” the method could be used.

The Department for Environment, Food and Rural Affairs (Defra) confirmed last night that an amendment to the Welfare of Animals (Slaughter or Killing) Regulations 1995 gave the authority for such a cull. A spokesman said that it was “not the method of choice” and would be used only if state vets had no other means to remove infected birds quickly.

In an epidemic, however, and if human infection were a possibility, Defra fears that some poultry workers may decline to help in the usual methods of culling — rounding up birds by hand, stunning them and humanely gassing them in a mobile chamber. The plan is certain to put Mrs Beckett and Ben Bradshaw, the Animal Welfare Minister, on a collision course with many Labour backbenchers and Opposition MPs.

Peter Ainsworth, the Shadow Rural Affairs Secretary, condemned the move as “barbaric”, and expressed anger about the secrecy in drawing up the new powers.

“Issuing emergency powers on a Bank Holiday weekend is not the way to go about business and we would not support a policy of deliberate suffering that is inhumane,” he said.

Philip Lymbery, the chief executive of CIWF, said: “Ventilation shutdown is likely to be no better than simply burying birds alive, and is therefore totally unacceptable on welfare grounds.

“Any outbreak of avian flu has got to be dealt with swiftly and efficiently, of course, but also humanely.”

John Avizienius, the RSPCA head of farm animals, said: “It’s hard to conceive of a scenario where this would be needed, but the RSPCA would not endorse ventilation shutdown as a humane method of slaughter. We also have concerns about how ‘exceptional circumstances’ would be interpreted.”

Peter Bradnock, the chief executive of the British Poultry Council, said that the industry had not been consulted on the powers. The Times has learnt, however, that approving death by suffocation during an epidemic may be an attempt to win approval for the gassing of entire chicken houses during a virulent avian flu outbreak. State vets and industry chiefs believe that gassing flocks would be the quickest and most humane form of slaughter if the deadly flu virus took hold.

Trials took place in Northern Ireland last autumn, when a combination of carbon dioxide and argon emerged as the fastest and kindest method of killing.

During the less virulent H7N3 alert last week a gassing trial took place in Norfolk, organised by the Scottish Executive. It is understood that state vets, the fire brigade and Health and Safety Executive officials monitored the use of gas in an empty shed. The aim was to find out how much gas would be needed to ensure that birds were killed, and how best the gas should be introduced into a unit.

The method was used in the Netherlands three years ago to wipe out most of 30 million birds kept on commercial farms producing chicken for human consumption.

Farmers and poultry companies have welcomed the trials, saying that culling birds quickly could be vital in an outbreak of avian flu.

About 118 million chickens are kept in commercial units; only about eight million are free range. There are also about another 30 million that are reared indoors for their eggs.

On some of the 2,000 farms in Britain there may be more than a million birds at any one time, with about 55,000 kept in sheds 300ft long and 80ft wide, and from 7ft high in the eaves to 15ft in the middle.

Explore World News

• World Agenda

• Iraq News

• US & Americas News

• Europe News

• Middle East News

• Asia News

• Africa News

• Ireland News

Times Recommends

• Fish and chip shops ‘selling cobbler as cod’
• T in the Park festival at Kinross
• How to get Madonna’s Thighs