Dealers looking to illegally export European eels from the UK have been exposed by BBC Countryfile.
Posing as a UK fisherman who had legally caught the eels on the River Severn in Gloucestershire, presenter Joe Crowley was approached by Chinese and Russian buyers and a UK exporter.
They were prepared to pay up to seven times the normal catch price if the eels could be sent out of the EU.
An export ban on the endangered species has been in place since 2010.
Organised crime gangs are said to be smuggling about 350 million live baby eels - or 'glass' eels - every year to Asia, where they are farmed and sold as a delicacy.
Andrew Kerr, of the Sustainable Eel Group said the illegal trade in glass eels, also known as elvers, was estimated to be worth about £3bn a year.
Mr Kerr told the BBC: "It's the most trafficked animal by number and by value.
"It leaves here at one Euro each and then one year later, having been grown in the 900 eel farms of inland China, it's worth 10 Euros - and that's pretty tempting.
"This is the greatest wildlife crime on the planet."
The illegal trade has previously been focused on stocks in France and Spain but now smugglers have turned their attention to the UK, where glass eels can only be caught by licensed fishermen.
Countryfile's investigations team posted an advert on an online trading website, offering live eels caught in the River Severn for sale.
One buyer from China offered more than £1,000 per kilo for the eels to be shipped to Malaysia, despite acknowledging that the export would be illegal.
The current price for eels bought and sold legitimately within the EU is about £150 per kilo.
Another buyer from Russia asked for the catch to be sent to Lithuania legally where he would then arrange for the eels to be moved over the border to Russia.
The team was also approached by a UK-based commodities trader who said he had a client in Asia who was looking for glass eels to be exported to South Korea.
When later confronted, he said he knew that it was illegal to export eels, that he did not have a buyer in South Korea and that he was only "speculating".
He added that he had never exported glass eels and had no intention of doing so.
Since the 1970s, the numbers of eels reaching Europe is thought to have declined by about 90%.
Today they are protected as an endangered species by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).
They are also named on the International Union for Conservation of Nature's Red List of Threatened Species.
But campaigners are concerned that a lack of monitoring is allowing glass eels to be moved between EU member states and beyond, with few traceability checks enforced or records kept.
Ian Guildford, of the National Wildlife Crime Unit, said it was often hard to convince other enforcement agencies to take the crime seriously.
He said: "This is major crime and, once we can get people to understand the severity of the problem, then we might get somewhere."
See the full story on Countryfile on BBC1 at 19:00 BST on 16 June and afterwards on the iPlayer.