The claims, which have been championed by the Daily Mail under the headline "Disturbing footage of pigs struggling to breathe as they're killed by CO2 stunning method being used by supermarket abattoirs", were made by activist group Eyes on Animals, which had filmed inside an unidentified pig slaughter operation somewhere in north-west Europe.
According to the Daily Mail, the footage shows "animals climbing over each other and gasping for breath as they enter the CO2 pits". It claimed that animals can "panic and kick out" for up to 30 seconds before losing consciousness.
The Mail goes onto highlight that the system of group stunning using CO2 is favoured by a wide number of UK supermarkets, and Tesco was calling on all suppliers to switch to the system by 2018.
However, The Humane Slaughter Association stated: "Increasingly, in larger plants in the UK and elsewhere, carbon dioxide is being used for the stunning and killing of pigs. For large operations with high throughput rates (eg 800 per hour), this is often the most reliable slaughter method for ensuring consistency in terms of good welfare and quality. Although the inhalation of carbon dioxide is aversive, overall controlled atmosphere stunning may have some welfare advantages.
"For the system to be as humane as possible, it is essential that animals are exposed to the maximum concentration of carbon dioxide as soon as possible and that the dwell time is sufficient to ensure that animals do not regain consciousness before death. All operators shackling and bleeding pigs should be capable of checking for, and recognising, signs of both effective and ineffective stunning. They must know what to do if signs of recovery are seen."
Welfare campaigners, however, are calling for alternative gas uses and mixes, such as argon, which are non-aversive and appear to cause no stress to the animals.
Stephen Rossides, director of the British Meat Processors Association, responding to the claims, said: "Gas stunning using CO2 is legally permitted and is generally regarded as 'best available practice' when carried out correctly. As an industry, animal welfare is at the forefront of what we do and we are always looking for ways to improve methods and conditions. According to the Humane Slaughter Association it may have some welfare advantages as well."
Lizzie Wilson, policy officer with the National Pig Association, said: "Our position on gas stunning of pigs with a high concentration of carbon dioxide is that it is a legally permitted method of inducing loss of consciousness prior to slaughter and currently represents a best available practice, when operated correctly and monitored carefully."