Sainsbury's and Harrods accused of fish con

The claim follows a nationwide survey by the Food Standards Agency, which has become increasingly concerned about food fraud.

Its researchers found one in ten supposedly wild sea bass and sea bream - among the most sought-after fish - were in fact farmed.

The proportion rose to almost one in seven of 'wild' salmon samples tested.

Fresh wild salmon usually costs at least 40 per cent more than its farmed equivalent. A similar pattern is seen with the price of sea bass and sea bream.

The survey, which looked at fish bought from the high street and auctions, suggests wild fish served up in restaurants may also have been farmed.

Food fraud is a rising problem. Recent examples include battery cage eggs from Europe being sold as free range and ordinary beef marketed as organic.

The FSA took 378 samples. Oil from the fish was then compared to a database covering authentic wild and farmed fish.

Of 59 samples of supposedly wild sea bass, six were definitely farmed while there was a question mark about one other.

Thirty five samples of 'wild' sea bream were tested. Four were definitely farmed and nine others questionable.

A total of 34 'wild' salmon were examined. Five were definitely wild while one was of doubtful origin.

Among those selling farmed salmon as wild were Harrods and a Sainsbury's store in Stroud, Gloucestershire.

Salmon sold at Tesco stores in Lancashire and Gloucestershire, a Waitrose in Kent, and a Morrisons store in Cumbria, had a question against it. These did not match the database definition for either wild or farmed salmon.

Supposedly wild sea bream sold by South West Seafoods in Plymouth and David Noblet in Lancashire was also farmed.

An Asda store in Gloucester was said to be selling wild sea bass which was farmed but this has been challenged by the company-which said all its sea bass was farmed and the pack tested was labelled correctly.

FSA head of Food Authenticity Dr Mark Woolfe said retailers should tighten up their procedures to ensure customers get what they are paying for.

A spokesman for Harrods said "human error" may have been to blame for farmed and wild salmon being mixed up.

He added that revised stock control measures have been put in place. Kensington and Chelsea Council has decided to take no further action because the store had made a "simple mistake".

Sainsbury's said it had launched an investigation, adding that each batch of Wild Alaskan Salmon was checked for traceability and chain of custody when it arrived in the country.

Consumer group Which? said it was "unacceptable" that customers were being deceived.

"Trading Standards must step in to tackle the offenders and prevent consumers from being misled," said its food campaigner Miranda Watson.