All that is left is a deserted shack, open to the four winds, in the middle of nowhere, its three rooms littered with filthy mattresses and rolled up blankets. There are any number of similar places on the highlands of Foggia province in the south of Italy.

The place must have looked even worse when Stanislav Fudalin, 51, arrived here one night in early January after an exhausting journey by minibus from Krakow. The windows were already broken and the door would not shut. He stayed for almost a month with eight other Polish workers, with neither running water, electricity nor heating, apart from the brushwood they collected to burn in the fireplace. From dawn to dusk they laboured in the fields, and all night they struggled to survive the cold, hunger and despair.

"I was mad enough even to consider walking home to Poland," Fudalin says with a smile. But it is difficult starting when you do not know where you are. When they arrived the "reception committee" - two Poles and a Ukrainian called Pedro - made it quite clear escape was not an option. Fudalin still remembers Pedro's threats and the beatings he meted out at the slightest excuse: "He said: 'I make the rules here. You're my slaves. If you try to leave, I'll track you down and kill you. You'll go back to Poland in a dustbin bag' ".

Fudalin and his mates nevertheless got away, partly by luck and largely thanks to the Polish consulate, which came to pick them up one night with a van. On July 18 their testimony, and evidence provided by 30 others, led the carabinieri to liberate 113 Polish farm workers, slaving under inhuman conditions in several neighbouring localities. The police arrested Pedro and 26 other suspects - Poles, Ukrainians, an Algerian and an Italian - all charged with human trafficking and slavery.

This was the first the Italian public had heard of any forced labour camps. The investigation, codenamed Promised Land and carried out jointly with the Polish police, uncovered a vast operation supplying illegal labour. It had been running for at least two years between the south of Poland - where about 20 traffickers are now behind bars - and the Foggia area.

In this part of Apulia, where millions of tonnes of tomatoes are harvested every summer, casual workers from eastern Europe have gradually replaced Moroccan pickers, but no one imagined they had been reduced to slavery.

They had no contract, suffered terrible hardship, humiliation and "worse still sometimes", adds a police officer. Lorenzo Lerario, from the anti-mafia prosecutor's office in Bari, has decided to reopen the cases of 14 Poles who died under suspicious circumstances over the past two years near Foggia.

Police dealt with each case in isolation and tended to assume the men died in fights or in accidents. But prosecutors now suspect the death rate was abnormally high for people who were mostly young and healthy.

Every summer for the past three years a mobile Médecins Sans Frontières (MSF) unit has been operating in the Foggia area. "We are facing the same situation again, with foreigners arriving in good health and falling ill here because of the foul conditions to which they are subjected," says Andrea Accardi, head of the MSF mission in Italy.

An MSF report published last year shows that the worst diseases are diagnosed on foreigners who have been here longest. More than a third of all immigrant workers live in derelict buildings. More than half have no running water, a third must do without electricity and almost half without proper sanitation. A third of the workers examined by MSF doctors had suffered ill treatment in the course of the previous six months.

The Promised Land investigation revealed the full scale of the problem. "It is not just a police matter," says Lerario, adding that the Bari prosecutors are handling the matter "very carefully and with all available resources, in close collaboration with the judicial authorities in Poland".

Domenico Centrone, the Polish consul for the Apulia region, says: "The local authorities did not want to know. I have been sending them letters for four or five years, particularly the chief of police in Foggia, but all to no avail. They just would not believe me."

It was the discovery, in August last year, of 90 Poles locked up in a labour camp at Orta Nova, south of Foggia, that finally prompted a response. Centrone, 49, who runs a food processing firm, subsequently accommodated and even employed several of the inmates, ultimately persuading them to file a complaint.

Many were so scared of reprisals they would not talk. Some of the gangs are still operating and the authorities advise journalists not to visit certain places without an escort.

Once the Poles had been moved to Castellana Grotte, a small town 150km south of Foggia, the bravest agreed to make an official statement. Apart from a few details, they all told much the same story: a small ad in a local paper or on the net promising a decently paid job (€5 or €6 an hour, with board and lodging), a loan from the bank or their family to cover travel expenses (€200 to €400), a minibus driving round and round in the Italian countryside till night fell, and finally their arrival in hell.

Marek Pajestka, 45, still looks scared stiff. He says: "At 4am the gangmasters took us off to an artichoke field where we worked for six hours, then on to a tomato field for another 10 hours, without any breaks." He worked 16 hours a day for the next two months.

For €6 an hour it might almost been worth his while. Pajestka, a burly unemployed coach driver, certainly needed the work. But when the exhausted workers reached the Orta Nova camp every night at 10.30pm they had to pay for everything. Only the beatings were free of charge. To spend the night in the house, with 60 other inmates, the price was €5. Alternatively they could sleep outside in a tent, on the hard ground, for €3. They were also charged for food (bread and pasta) and candles (there was no electricity). They even had to pay €1 every morning for the ride to the fields. If they fell ill it cost them €20 for every day not worked.

"I lost all hope of ever getting paid the day six of them laid into a big lad who was demanding his pay," says Pajestka. "After that they dragged him out of the camp, all covered in blood, and dumped him beside the track. They were shouting: 'You've got two hours to clear out, otherwise we'll kill you'."

Jakub Olszewski, 21, landed up in the same hell-hole. In three weeks Olszewski lost seven kilos. He had given up the idea of ever escaping when the carabinieri turned up. "There were no latrines and only two showers . . . more like pipes . . . for about 100 of us," he says.

Pajestka and Olszewski describe a compound, surrounded by walls topped with barbed wire. At night the gate was padlocked. The only light in the vicinity came from a nearby house where the gangmasters were based. All the workers - Poles, Romanians, Africans and Italians - refer to these men as "capos". The caporalato is as old as farming itself in an area dominated by vast properties, where the bosses never deal directly with their labourers. Capos have always hired day-workers, driven them to the fields, paid them and taken them home in the evening. They are an integral part of a system based on illegal employment. But with the disappearance of Italian pickers, their place taken by foreign workers, the gangmasters' methods have become increasingly brutal in the battle to harvest the crop of "red gold".

The carabinieri admit they are not up to the task. "How are we to keep a check on invisible labour that's constantly on the move?" asks one of the nine men at the station in Ascoli Satriano, pointing to a map of the 334-square-kilometre locality.

The day of our visit the commander of the Ascoli carabinieri met a Polish couple who had had no news of their son, Gregorz Kusz, 28, for two years. The authorities in Warsaw have registered 119 missing people who have travelled to Italy since 2000. Half of them are thought to have disappeared in the Foggia area. Polish police have now posted the names and photographs of these men and women on their website and the embassy in Rome has officially submitted the list to the Italian prosecutors.

The Kuszes have left a photocopy of their son's passport at the police station in Ascoli, as they have done elsewhere, but there is little chance of success.

"How can we be expected to look for people with no proof they ever arrived?" asks a prosecutor. However the increased publicity does seem to have stirred the office of public prosecution in Foggia into action. Cases are being reopened and evidence from different sources correlated.

Some of the phone conversations recorded during the Promised Land investigation suggest there is plenty of cause for alarm. Police overheard one gangmaster telling his girlfriend how angry he was about two workers who had escaped. He said: "I won't allow them to behave like that. Today I'll go along to the fields and kill a couple, just to set an example."

Olszewski only saw fellow workers being beaten and kicked. He says: "Some of the people missing may have been killed, but I reckon most are still alive and working somewhere else. They borrowed money from their families to come here. They can't go straight home after such a failure." He smiles faintly and adds: "Maybe they died of shame."