Wal-Mart, the controversial retailing giant, is under investigation in the US over allegations it is trying to pass off non-organic foods as organic.
It has been accused of using misleading labelling that is "tantamount to consumer fraud" by an organic farming watchdog, the Cornucopia Institute. The body has handed its complaints to the US Department of Agriculture (Usda).
The Wisconsin Department of Agriculture, Trade and Consumer Protection is also conducting an investigation into whether Wal-Mart is placing "natural" produce on shelf space labelled as containing organic items.
The Cornucopia Institute claimed to have found dozens of examples of Wal-Mart's mislabelling products - from "all- natural yogurt" to soya milk "made from organic soybeans".
Usda is examining the watchdog's claims, while the Wisconsin authorities are examining Wal-Mart's practices and those of other supermarkets within the state.
Wal-Mart said it had written to store managers to ask them to be careful, but added that consumers could tell whether a product was organic by looking for the Usda label.
The Cornucopia Institute first raised the issue with Wal-Mart last autumn, but found fresh examples of mislabelling on return visits to stores this month.
"The vast majority of organic farmers and food marketers operate with a high degree of organic integrity," said Tom Willey of T&D Willey Farms of California, an organic producer of fresh market vegetables. "These abuses endanger the credibility of the organic label for all of us."
Wal-Mart, which owns Asda in the UK, is trying to build a green reputation and attract more affluent consumers.
A year ago the retailer announced that it would stock new ranges of organic food.
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