Leading supermarkets and a top London store were among retailers yesterday identified as misleading consumers after selling "wild" fish that in reality was farmed, while others were criticised for failing to provide accurate labels on how and where their fish was produced.

Scientific tests by the Food Standards Agency on premium-priced fish sold by Sainsbury's, Asda and Harrods revealed that some of the animals were from farms and had not been wild. The snapshot survey in 15 local authority areas across Britain found that 15% of supposedly wild salmon, 11% of sea bream and 10% of sea bass were farmed.

Consumers typically pay double the retail price for wild fish, reputed to have health benefits as well as a better flavour.

The tests were done on fish bought anonymously at supermarkets, fishmongers, fish auctions and specialist food shops. Oil extracted from the samples, as part of a pioneering testing regime, revealed whether the fish were wild-caught or had been farmed.

The FSA's 40-page report revealed that one sample of sea bass from an Asda store in Gloucester, one salmon (wild Alaskan) sample from a Sainsbury's store in Stroud, and one salmon sample from Harrods in London, which were all sold as wild, had come from farms. Trading standards officers collected the 128 fish samples from retail outlets across Britain.

Another part of the survey found that 15% of retailers were in breach of labelling regulations by giving either incorrect advice or no information about the production method and geographic origin of the products.

Smaller businesses, such as independent fishmongers, scored least well in this area, although supermarkets such as Tesco, Morrisons, Marks & Spencer and Aldi, were also named for falling short of correct labelling. The information checks on the product labels were done by 15 UK local authority officers, between November 2005 and January 2006.

The FSA said in the report there was a legal requirement to state the production method. It added: "Increasing numbers of consumers wish to know about the origin of the food they buy. In addition, there is a large price differential between farmed and wild fish. Correct information on the production method to consumers is important to prevent food fraud."

Mark Woolfe, head of the food authenticity branch at the FSA, said: "Although this survey only gives us a snapshot of the market at the time the samples were taken, it does show areas where retailers can tighten up their procedures so that people both know and get what they are paying for."

Local authorities, who participated in the survey, would follow up on the findings, the FSA said. The agency itself plans to issue a guide for small businesses on fish labelling rules.

Sainsbury's said in a statement: "We take this report very seriously and have launched an investigation. [All] wild Alaskan salmon is checked for traceability and chain of custody as soon as it arrives in this country. As the product is certified as sustainable by the Marine Stewardship Council, traceability must be particularly robust, and the system is independently checked and audited by the MSC.

"A possible explanation for the variation in analysis/flesh composition is the diet of the fish in the wild. Wild salmon are omnivorous, so there is every possibility that at times their diet may consist of high levels of vegetable matter."

A Harrods spokesman said: "Four out of the five fish samples tested last summer were proven to be wild. Purchasing records show that both farmed and wild salmon were purchased and made available for sale at the time of the sampling. It appears that on the day the sample in question was purchased, human error may have been responsible for farmed and wild salmon ... being mixed up."

Asda said: "We simply shouldn't have been included in the FSA's report. We don't sell any wild sea bass in our stores and haven't done for years. All our sea bass is clearly labelled as farmed."

The National Federation of Independent Fishmongers said of the labelling: "We do not believe that it is a case of consumers being deliberately misled. Many retailers are not given the appropriate catch areas or production information by suppliers."

The offenders

Sold as wild, found to be farmed

Asda Gloucester Sea bass

Sainsbury's Stroud Wild Alaskan salmon fillet

Harrods London Salmon

Incorrect or partially correct labelling

Aldi Bridgwater Premium smoked salmon

Tesco Camborne Haddock loin fillets

Tesco Ipswich Plaice fillets, whole mackerel

Tesco Westcliff on Sea Trout fillet, whole rainbow trout

Marks & Spencer Pontypridd Wafer-thin Scottish smoked salmon, Scottish oak smoked salmon

Morrisons Paignton Scottish salmon fillet, Scottish salmon steaks

· Read the Food Standards Agency report in full