A passenger has reportedly been caught trying to smuggle 600,000 live eels onto a plane from Heathrow Airport.
Border Force officials found the eels hidden under a load of chilled fish before a flight bound for Hong Kong took off on 15 February.
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The European 'glass' eels are an endangered species, but are seen as a delicacy in the Far East and had a street value of around £1.2 million.
According to the Evening Standard, a 64-year-old man was arrested in Chessington in connection to the incident and was bailed until August.
The eels were shipped back to Spain, where they had originally come from.
Grant Miller, of the national Border Force CITES team at Heathrow, said: "This was a significant seizure, the first of its kind for this type of species at export in the UK.
"Due to their conservation status, it was important to repatriate them alive and well."
UK eel fishermen need to be registered, but European eels can be legally fished, exported and eaten within the European Union. However export to countries outside the EU is banned.
"Their diligent work to protect wildlife from illegal trade is vital and this particular seizure demonstrates that some of our most vulnerable wildlife, in this case an endangered species of eel, are constantly under threat from those that wish to benefit financially from animals that need our protection."