Asda's £2 chickens came under fresh scrutiny from the press last week after Compassion in World Farming named it the worst UK retailer for animal welfare.

Asda sits at the bottom of the list - which is topped by M&S - following the findings of CIWF's flagship biannual survey of animal welfare practices in retail.

But Asda is crying foul and pointing an accusatory finger at the CIWF survey methodology. The survey was based on responses from the retailers , but Asda refused to co-operate, claiming the questions were biased towards smaller retailers. Its ranking has instead been calculated from the same figures it provided in 2005.

The survey is based on answers to 85 questions about the supermarkets' commitment to animal welfare and awards a rating out of five stars across various categories. Questions range from how long dairy herds are kept outside to the type of bedding used for pigs.

But Asda slammed the survey as subjective and biased , pointing out that other retailers that failed to respond, including Iceland, Budgens and Lidl , have been spared a ranking.

"The survey favours smaller retailers such as M&S and Waitrose because it doesn't take into account other things that bigger retailers are doing," says Asda's head of agriculture Chris Brown. "Just because we haven't completed the questionnaire does not mean we don't take welfare seriously."

Brown points to the fact that the methodology penalises retailers if they don't have targets for using cage-free eggs as an example. Asda has no such targets yet the growth in sales of free-range eggs at the retailer over the past 12 months is the equivalent to total sales of free-range eggs in M&S, he says.

Asda believes a better assess ment is to look at the outcomes of how animals are treated. "A lot of these measurements are about inputs, none of them define animal welfare outcomes," says Brown. "For example, what level of stock density do chickens want? We need to make animal welfare claims based on what animals need, not on our assumptions of what we think they need."

The RSPCA supports Brown's claim. Until now it has looked at government statistics on animal welfare but, under its Freedom Food scheme, it is visiting farms to assess animal welfare, says head of external affairs David Bowles. " We are now trialling a scheme in Freedom Food farms where a farm welfare assessment can judge how content an animal is - it will provide for the first time very good data on animal welfare.

"There are a lot of assumptions, such as 'free-range is good and intensive is bad', but this way we will be able to compare the welfare of an animal across these systems. The animal will tell us its welfare instead of a scientist or a campaigner."

European-wide assessments could soon be commonplace, providing much more robust information on animal welfare that no one will be able to refute, Bowles adds.

CIWF director Philip Lymbery defends the methodology, however. "The survey was not something developed on the back of an envelope," he says .

"It has been put together in clear collaboration with stakeholders. Consumers should know about the retailers' welfare policies and we facilitate that so that companies can make progress and get recognition. "

And other retailers are happy with the approach. "High animal welfare is at the heart of our business," says M&S director of food Steve Esom.