Millions of tuna portions are reportedly being dyed pink to make them appear fresh.
Fraudsters are reportedly colouring the low-quality cuts to mask them as premium fish.
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And, according to official figures, the underground market is making scammers hundreds of millions of pounds per year.
But the cuts could easily find their way across the continent, experts have warned.
According a European Commission document seen by the industry magazine IntraFish, consumers across the EU are paying £174million for fish that is illegally treated with vegetable extracts containing high levels of nitrites.
According to official figures, the underground market is making scammers hundreds of millions of pounds per year
This gives the fish the purple reddish hue of high-quality tuna.
Reports suggest five million tuna portions - equivalent to 500 tonnes - are being altered to appear fresh.
This potentially doubles their value price, and the practice is dangerous given the fact tuna is not eaten raw.
The portions could also contain high levels of histamines that can cause people to suffer allergic reactions.
Enrico Brivio, EU spokesperson for Health, Food Safety, Environment, Maritime Affairs and Fisheries, told IntraFish that action was being taken to stop the practice across the bloc.
"There is a close collaboration between the EU Food Fraud network and the fish industry to stop this alleged illegal practice," he claimed.