Some of Britain's major food retailers are guilty of misleading customers by printing unfounded sustainability claims on certain fish products, an environmental law group alleged today.

ClientEarth made the charges in a report published just a week after the Marine Stewardship Council (MSC) was accused of "duping" consumers by awarding its eco-label to fisheries with falling fish stocks.

Although some good practice was identified, ClientEarth's investigation found that 32 of 100 labels at nine supermarkets, including Tesco, Asda, Marks & Spencer, Sainsbury's and Waitrose, had unverified or misleading claims on sustainability or protecting the marine environment.

The report said 22 of these claims were unsubstantiated by the information provided by the retailers, while 10 had no evidence at all to allay ClientEarth's concerns they were misleading.

It blamed a lack of comparability and consistency, with different supermarkets using different criteria, for "a confusing landscape that does not allow consumers to make informed choices about the environmental impacts of their supermarket choices".

The report also voiced concerns that labels failed to convey that many popular types of fish products came from threatened stocks, or were caught by methods that could damage the environment or other marine species.

For example, ClientEarth felt that while Sainsbury's claim that its Taste the Difference haddock loins were line-caught in Norwegian and Icelandic waters recognised that certain fishing methods could impact on the environment, it was still of concern because "the fish comes from areas where haddock stock health is at risk due to high fishing levels".

James Thornton, ClientEarth chief executive, said EU standards for environmental claims about fish products should apply in the same way as for the term 'organic' and called for supermarkets to remove sustainability labels until their claims could be proven.

"It would be shocking to find out that the free-range chicken you bought was actually battery farmed," he said. "Discovering the fish you're eating, which is labelled as responsible or environmentally friendly, actually led to the deaths of threatened species also leaves a bad taste in the mouth.

"We would like all supermarkets that have misleading claims on the products we've identified to remove them as soon as possible, or to prove them with evidence."

He warned that if the supermarkets fail to remove the offending labels, "complaints can and will be made to the Office of Fair Trading, arguing breaches of consumer protection laws".

In an emailed statement, Sainsbury's welcomed ClientEarth's call for consistency of sustainability claims, but insisted that the products identified in the report were correctly labelled.

"We do not carry on pack claims in relation to sustainability unless the fishery is independently certified as sustainable by the MSC. MSC do not have standards for farmed fish and there is no other equivalent standard, so we do make claims about sustainability on farmed fish. We are however, happy to make claims about responsible sourcing where suppliers meet the requirements of our sustainability rating system," it said.

In respect to the allegations about its haddock, it added:

"The Icelandic government changes its total allowable catch annually to reflect the latest stock information. The last change was made in September 2010 when they changed the allowable catch for the haddock fishery making it consistent with the precautionary approach in line with the latest data. Consequently, we believe this reflects responsible management in that fishery and justifies our "responsibly fished" claim."

The retailer also maintained that allegations made over its Taste the Difference Jumbo King Prawns were based on insufficent information.

M&S was also approached, but declined to comment.

ClientEarth is also involved with Fish Fight, TV cook Hugh Fearnley-Whittingstall's collaboration with Channel 4 to stop the practice of discarding fish under the Common Fisheries Policy (CFP), which launches this month.