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.

“I thought I was going to die,” said Vuthy, a former monk from Cambodia who was sold from captain to captain. “They kept me chained up, they didn’t care about me or give me any food … They sold us like animals, but we are not animals – we are human beings.”

Another trafficking victim said he had seen as many as 20 fellow slaves killed in front of him, one of whom was tied, limb by limb, to the bows of four boats and pulled apart at sea.

“We’d get beaten even if we worked hard,” said another. “All the Burmese, [even] on all the other boats, were trafficked. There were so many of us [slaves] it would be impossible to count them all.”

CP Foods – a company with an annual turnover of $33bn (£20bn) that brands itself as “the kitchen of the world” – sells its own-brand prawn feed to other farms, and supplies international supermarkets, as well as food manufacturers and food retailers, with frozen or cooked prawns and ready-made meals. It also sells raw prawn materials for food distributors.

In addition to Walmart, Carrefour, Costco and Tesco, the Guardian has identified Aldi, Morrisons, the Co-operative and Iceland as customers of CP Foods. They all sell frozen or cooked prawns, or ready meals such as prawn stir fry, supplied by CP Foods and its subsidiaries. CP Foods admits that slave labour is part of its supply chain.

“We’re not here to defend what is going on,” said Bob Miller, CP Foods’ UK managing director. “We know there’s issues with regard to the [raw] material that comes in [to port], but to what extent that is, we just don’t have visibility.”