Asda has launched an astonishing attack on its rivals' animal welfare practices - claiming they submitted false statements for a recent animal welfare survey.

The accusations come after Asda was named as the worst retailer for animal welfare by Compassion in World Farming. The survey, of eight supermarket chains, relegated Asda to the bottom of the list after CIWF used figures the retailer had supplied in 2005, while other stores were rated on 2007 data.

Asda had boycotted the survey because it said it was biased towards smaller food retailers, whose limited demand for meat was easier to fulfil - and has subsequently questioned the veracity of some of the claims made in the survey by its rivals.

"The CIWF's assessment of the results has not been terribly thorough," said Chris Brown, Asda agricultural manager.

"Some of the statements in this survey are about as honest as a Labour party donation list."

Brown questioned answers from Sainsbury's and Morrisons that he didn't believe to be true.

"Sainsbury's says 85% of its beef cows have access to the outside all year round, so presumably they are all eating Taste the Difference mud in the winter," said Brown.

"Morrisons buys 30% of its cattle at auction so how can it come up with a figure that 70% of its cattle have outdoor access day and night if it doesn't know where all of its animals come from?"

Morrisons declined to comment. But a spokeswoman for Sainsbury's, ranked fourth in the survey, and which was also named Most Improved Supermarket and Best Volume Retailer, said: "We are thrilled that the work we have put into improving animal welfare was recognised.

"We have a long-standing, open and honest relationship with CIWF, and it is confident that the information we supplied to it on our current animal welfare performance was accurate."

The CIWF insisted it had taken steps to check the data used for the research.

"The whole premise of the survey is done on trust and honesty on the basis that the results are being placed in the public domain for public scrutiny, so the emphasis is on being honest," said chief executive Philip Lymbery. "We do take reasonable steps to sanity- check the answers."

The CIWF also defended its decision to include Asda in the rankings despite using old figures for the supermarket. "As a big four retailer the whole league table would lack a very significant point of interest if Asda were not represented," said Lymbery. "Asda didn't deliver any answers this time round, which was regretful, but we have clearly flagged up that the data we have used is from 2005."CIWF ranking

1. M&S

2. Waitrose

3. The Co-operative Group

4. Sainsbury's

5. Tesco

6. Morrisons

7. Somerfield

8. Asda - 2005 figures

NB - Aldi, Asda, Budgens, Iceland, and Lidl did not supply figures