One fish has been reared in the chilly fathoms of the Atlantic Ocean, and its meat is firm and flakes satisfyingly when cooked. The other is farmed in the rivers of the Mekong delta in Vietnam, from where it is frozen and imported to Britain. Although white, its meat crumbles.

To connoisseurs they are as divergent as chalk and cheese. Yet in more and more of Britain’s 11,000 fish-and-chip shops they are the cause of a culinary scandal as some unscrupulous friers — albeit a minority — try to pass off the Vietnamese river cobbler as traditional British cod.

The cobbler — Pangasius hypophthalmus — is one of 20 types of catfish produced in Vietnam for 15 years. It is frozen and exported to Britain, where it is also sold as panga, basa and tra. Its appearance in supermarkets — and in fish-and-chip shops — is not the cause of controversy. The Vietnamese cobbler has won admirers at dinner tables and chip-shop counters.

Once dipped in batter and deep fried, however, trading standards officers are reporting increasing instances of the river cobbler being sold as cod in one of the more unusual manifestations of the recession’s impact on our lives.

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For fish-and-chip shop operators, the cash differences — and the temptations — are huge. The wholesale price of river cobbler this weekend was £5 a kg compared with £11.75 a kg for cod.

The first case of fraud was found at the Cat Hill fish bar in Bromsgrove, Worcestershire. The owner was fined £8,000 plus costs, reduced to £4,000 plus costs after an appeal.

Charges were brought under the Food Safety Act under which the maximum penalty is £20,000 and/or six months in prison. More prosecutions are in the pipeline in the West Midlands and checks are being carried out at other fish-and-chip shops.

John Dell, head of enforcement at Worcestershire Trading Standards, said: “We discovered the fraud after a woman contacted us convinced that the fish she had been served wasn’t cod. We conducted tests and eventually found it was pangasius.

“There is nothing wrong with this fish and it is widely available here in supermarkets and restaurants. People have been looking for more sustainable fish species to protect cod stocks. But if pangasius is sold, it has to be labelled as such. It cannot just be substituted for cod. That is not fair on the customer or to other local businesses in competition with the fraudster.”

Mr Dell has reported the issue to the Office of Fair Trading to alert other enforcement agencies that the scam is likely to be happening nationwide.

In an interview to be broadcast tomorrow in a BBC One documentary, What’s Really In Our Food?, he says that investigations into food fraud have risen by 20 per cent in the past year. “The recession is having a big influence on food fraud. We’re finding a lot of things wrong in the food area. Chip shops have been conning people,” he said.

The National Federation of Fish Friers is angry that the reputation of its 8,000 members is being tarnished by the scam. Douglas Roxburgh, its president, said: “We’re aware of these frauds. The authorities have got to hand out heavier fines. I’d have said it should be £10,000 at least to set an example. It is misleading the public and mislabelling, and I say throw the book at them. Someone is just trying to make a fast buck.”

Many fish-and-chip operators have started stocking pollock and pangasius as an alternative to cod, he said, but they are sold as pollock and chips or panga and chips.

“We can’t support mislabelling. If customers are dissatisfied with their meal, the next time they go for a takeaway they’ll remember the poorer cod and chips and go for a pizza or a burger instead. It’s very damaging for our business.”

Sarah Appleby, head of enforcement at the Food Standards Agency, said that the watchdog was aware that pangasius was being substituted for cod, especially when prices fluctuated.

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