"The recall is necessary because the label is very important. Consumers are entitled to the right information," FDA secretary general Pakdee Pothisiri said.

He ordered the recall immediately after product tests confirmed a complaint lodged by Asst Prof Apinun Suvarnaraksha, who has taught at Maejo University's Faculty of Fisheries Technology and Marine Resources.

Pakdee said the FDA would take legal action against the offending manufacturers on many counts relating to the false information on the labels. The companies' executives could be sentenced to 10 years in jail and fined Bt100,000.

"We have worked closely with the Thai Food Processors' Association all along and yet the canned-fish companies have never informed us about the change in their contents," Pakdee said.

Surin Tohtabtiang, president of Kuang Pei San Food Products Plc, blamed a sardine shortage for the switch. Mackerel was in fact a more expensive fish and contained more calcium, he said.

"The point is this is against the law," Pakdee said, adding that none of the manufacturers had reported low supplies of sardines to the FDA. Apinun had found that several brands did not use sardines as claimed on their labels.

The FDA regularly tests products in the market but has never checked the type of fish used in the cans before, he said.

A food-shop owner in Bangkok said many of her customers ordered dishes made of canned fish and none ever asked whether it was sardines or mackerel. "At the moment we are feeling upset that the manufacturers didn't tell us the truth," she said. "It's common sense that if you're a seller, you will want to boast about your product's better quality. If it's better, why don't you tell consumers?" she said.

In Chiang Mai, company employee Kittipoj Ruangnarong, 26, said the FDA should learn a lesson. "This highlights the fact that the FDA fails to ensure that food firms abide by the regulations. It's time that the FDA improved its quality inspections for all products under its jurisdiction," he said.