Hong Kong's Centre for Food Safety says it collected noodlefish samples for testing, and results show that nine of the fifteen samples taken contained formaldehyde at levels ranging from 160 to 620 mg/kg.

"Although formaldehyde can be found naturally in marine products such as Bombay-duck and crustacean, up to several hundred mg/kg, the one detected in the noodlefish samples was believed to have been added on as a preservative after the fish were caught, or during transportation or storage," a CFS spokesman said.

Under existing legislation, formaldehyde is not permitted for use as a food preservative. Contravention of the law could lead to a maximum fine of HK$50,000 and six months' imprisonment. The CFS called on the trade not to add formaldehyde into fish or other marine products.

"At the levels of formaldehyde detected in the noodlefish samples, normal consumption should not pose any adverse health effects," the spokesman said.

"We have asked the concerned fish stalls to stop selling noodlefish, and are investigating the source of the fish in question."