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Slaves forced to work for no pay for years at a time under threat of extreme violence are being used in Asia in the production of seafood sold by major US, British and other European retailers, the Guardian can reveal.

A six-month investigation has established that large numbers of men bought and sold like animals and held against their will on fishing boats off Thailand are integral to the production of prawns (commonly called shrimp in the US) sold in leading supermarkets around the world, including the top four global retailers: Walmart, Carrefour, Costco and Tesco.

The investigation found that the world’s largest prawn farmer, the Thailand-based Charoen Pokphand (CP) Foods, buys fishmeal, which it feeds to its farmed prawns, from some suppliers that own, operate or buy from fishing boats manned with slaves.

Men who have managed to escape from boats supplying CP Foods and other companies like it told the Guardian of horrific conditions, including 20-hour shifts, regular beatings, torture and execution-style killings. Some were at sea for years; some were regularly offered methamphetamines to keep them going. Some had seen fellow slaves murdered in front of them.

Fifteen migrant workers from Burma and Cambodia also told how they had been enslaved. They said they had paid brokers to help them find work in Thailand in factories or on building sites. But they had been sold instead to boat captains, sometimes for as little as £250.