The migrants typically earn €25 a day and are recruited by gangmasters working for farm owners

It is perhaps the worst address in Western Europe. A ramshackle slum with a noisy road on one side, a railway on another, and a stagnant-looking river flowing nearby.

The camp itself consists of little more than a collection of shoddily-erected canvas tents and some abandoned buildings and sheds. Behind the wire fence, fires burn amid piles of rubbish – discarded wholesale-sized tins of olive oil, plastic bottles, food scraps and other unidentifiable filth.

The migrants milling around are in Rosarno in Calabria, southern Italy, to harvest the region's extensive orange crop – at nearly 900,000 tonnes, the second largest in the country. Dozens of them swarm around – cooking, chopping firewood and trying to keep warm.

They are from Africa – Ghana, Burkina Faso, the Ivory Coast – and this squalid camp, where doctors treating skin conditions linked to the migrants' work say conditions can be as bad, or worse, than in war zone refugee camps, is currently home to at least 200 itinerants.

Each winter, as many as 2,000 migrants travel to this agricultural town to scratch a living picking oranges that will end up on sale in markets and supermarkets, or as juices or concentrates used in the manufacture of soft drinks sold in Italy and across Europe.

But these fruit products could be linked to a life of squalor and exploitation for some of those working at the bottom of the supply chain, an investigation by The Ecologist has revealed.

Campaigners are calling on multinational food and drink firms purchasing orange ingredients from the region to help address the problem, claiming the nature of the current supply chain – with processors sourcing from multiple co-operatives and farmers – and the widespread use of migrant workers make it difficult for companies sourcing from the region to avoid procuring "tainted oranges".

Italy's largest farmers' association says it has has written to several companies – including Coca-Cola, manufacturer of Fanta – complaining that prices paid for orange concentrates are unfair and foster unpleasant conditions. Coca-Cola says a letter it has been shown referred to another company's products. Coca-Cola, which sources oranges in Calabria, denies any wrongdoing but admits the nature of the supply chain means it is unable to audit every farm or consortium whose juice may be bought by its supplier.

There are thought to be around 50,000 migrants currently existing as seasonal croppers across Italy. They typically earn €25 (£21) for a day's work in the orange groves and are often recruited by gangmasters who typically charge workers €2.5-€5 for transport and sometimes make other deductions. Diallo, from Guinea, who has been prominent in trying to raise the plight of the workers with politicians, is blunt. He said: "I tell them, we're not criminals, I am working, they [the farmers] are exploiting us. We don't have nobody to help... [this is] apartheid, colonisation, silent colonisation, silent slavery. There's no future."

One farmer who employs migrant workers, Alberto Callello, maintains workers get a reasonable deal and blames the economics of orange farming and the wider supply chain for the conditions. He said: "Twen-ty-five euros is the minimum wage. It is a poor wage but it is a poor economy. Poor, but not exploitation."