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 Thailandare 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."

The supply chain works in this way: Slave ships plying international waters off Thailand scoop up huge quantities of "trash fish", infant or inedible fish. The Guardian traced this fish on landing to factories where it is ground down into fishmeal for onward sale to CP Foods. The company uses this fishmeal to feed its farmed prawns, which it then ships to international customers.

The alarm over slavery in the Thai fishing industry has been sounded before by non-governmental organisations and in UN reports.

But now, for the first time, the Guardian has established how the pieces of the long, complex supply chains connect slavery to leading producers and retailers.

"If you buy prawns or shrimp from Thailand, you will be buying the produce of slave labour," said Aidan McQuade, director of Anti-Slavery International.

The Guardian conducted dozens of interviews with fishermen, boat captains, boat managers, factory owners and Thai officials in and around various ports in Thailand.Thailand enjoys a prime position as the world's largest prawn exporter in a vast seafood-export industry estimated to be worth some $7.3bn. Through multinationals such as CP Foods, Thailand ships out roughly 50,000 tonnes of prawns every year – nearly 10% of which is farmed by CP Foods alone.

Although slavery is illegal in every country in the world, including Thailand, some 21 million men, women and children are enslaved globally, according to the International Labour Organisation. These people may have been sold like property, forced to work under mental or physical threat, or find themselves controlled by their "employers". Thailand is considered a major source, transit and destination country for slavery, and nearly half a million people are believed to be currently enslaved within Thailand's borders. There is no official record of how many men are enslaved on fishing boats. But the Thai government estimates that up to 300,000 people work in its fishing industry, 90% of whom are migrants vulnerable to being duped, trafficked and sold to the sea. Rights groups have long pointed to Thailand's massive labour shortage in its fishing sector, which – along with an increased demand from the US and Europe for cheap prawns – has driven the need for cheap labour.

"We'd like to solve the problem of Thailand because there's no doubt commercial interests have created much of this problem," admits CP Foods' Miller.The Guardian's findings come at a crucial moment. After being warned for four consecutive years that it was not doing enough to tackle slavery, Thailand risks being given the lowest ranking on the US state department's human trafficking index, which grades 188 nations according to how well they combat and prevent human trafficking.

Relegation to tier 3 would put Thailand, which is grappling with the aftermath of a coup, on a par with North Korea and Iran, and could result in a downgrade of Thailand's trading status with the US.