The Scottish salmon farming industry faces a sales ban and fines after traces of a cancer-risk chemical was discovered in the fish.
Malachite green, a cheap disinfectant treatment for eggs and young fish, was banned in June 2002 but traces of the chemical are still being found in up to 19 per cent of farmed salmon.
The EU has threatened legal proceedings against Britain that could result in a ban or a fine unless the contamination is stopped.
The industry is still reeling from a U.S. study published last week, suggesting people should not eat Scottish farmed salmon more than three times a year.
It identified a higher cancer risk due to the salmon being contaminated with industrial and agricultural pollutants given to them in suspect feed. Malachite green, which is actually a fabric dye, has been used by fish farmers for decades as a treatment for dealing with parasites and fungal infections.
The salmon and trout farming industries have routinely used the chemical in the past because it is much cheaper than officially approved medicines prescribed by vets.
However, there are concerns that malachite green – and the metabolised chemical leucomalachite green – are dangerous toxins.
The presence of the toxins has remained relatively constant despite the 2002 ban. Official monitoring puts contamination levels at 20 per cent of fish in 2001 and 15 per cent in 2002. It was 19 per cent in the first three months of last year, although the average for the year is thought to have been around 15 per cent.
The Salmon Farm Protest Group spokesman, Don Staniford, said: "I would not be surprised if some Scottish fish farmers were still using malachite green.
"Consumers should not be subjected to such a suspect substance."
Scottish Quality Salmon, which represents three-quarters of the country’s fish farmers, said none of its members use the chemical.
It said positive tests were being found in very few adult fish, which may have been legally treated with the chemical while young some two years ago. It dismissed the allegations as "scaremongering" and "completely without foundation".