Official tests found a banned chemical in farmed salmon bought on Tyneside.
Scottish fish farms were criticised in a US study
The Veterinary Medicines Directorate (VMD) discovered traces of malachite green in Scottish farmed salmon bought at a branch of Morrisons in Newcastle.
The chemical, normally used to dye fabric, was also used to kill fish parasites. It was banned in 2002 amid fears it may damage human cells.
The Food Standards Agency described the find of the chemical in fish for human consumption as "unacceptable".
VMD discovered malachite green at 4.9 parts per billion - more than double the European Commission's limit of two parts per billion - in one salmon product out of 92 analysed.
The salmon was bought in February from the fresh fish counter at a branch of Morrisons in the Byker area of Newcastle.
The VMD insisted the level of malachite green found did not raise health concerns.
But a spokesman for the Food Standards Agency said: "The use of malachite green in aquaculture was banned in 2002 and its continued presence in fish for human consumption is unacceptable.
"Our understanding is that the VMD has taken follow-up action with the retailer and producer".
Dr John Webster, scientific adviser to Scottish Quality Salmon, which represents the interests of farmed salmon producers, said: "If there are examples of malachite green occurring in farmed salmon then something is wrong, although I'm sure it occurred in negligible quantities."
In a statement, Morrisons said: "We would like to reassure our customers that salmon is safe and healthy to eat.
"Morrisons use approved suppliers for this product who we believe to have rigorously followed all regulations relating to malachite green.
"Indeed, we are aware that the supplier ceased using it before the ban in June 2002.
"Tests carried out by the supplier, and by other independent authorities, have proved negative."
Earlier this year the Scottish Executive was asked to examine claims that farmed salmon was unsafe.
US scientists claimed Scottish fish was tainted with chemicals and urged people should limit their consumption of farmed salmon.