The Brazilian employment ministry said its officials raided 183 farms, the highest number since Swat-style teams were introduced 10 years ago. In total 4,133 workers were freed, with R$7.4m (£1.8m) paid to victims, the ministry said.

"What we know about is the tip of the iceberg," said Father Ricardo Rezende, an anti-slavery campaigner and author of Stepping Out of the Shadow: Slavery for Debt in Contemporary Brazil, the first study of modern-day slavery in Brazil.

Although slavery was officially abolished in Brazil in 1889, Fr Rezende said the official estimate of just 25,000 slaves in Brazil could be way off the mark: "The real figure could be 250,000."

Brazil's modern-day slave trade began to boom again during the 1964 military dictatorship. Following the creation of Sudam, a regional development agency which opened up industry in the Amazon region in 1966, business people and cattle ranchers flocked to the rainforests to make money. The landowners employed middlemen, known as gatos, who found unemployed workers in Brazil's impoverished rural communities, often in north-eastern Minas Gerais.

"The gato arrives in a community and says he can get legal work in a certain place," said Fr Rezende. "They say the person will be able to send money home to their family ... [but] when they arrive they discover they owe money for the transport and the food. The first imprisonment is that of the soul.

"Often a worker will have the false sense that he is in the wrong [if he flees]."

Neide de Oliveira was the first prosecutor in Brazil to secure a conviction against a farmer for using slave labour in 1998. She says landowners manipulate workers, knowing that slavery is, for many, a more attractive proposition than unemployment.

"I once interviewed a worker with malaria and the farmer had given him a paracetamol and told him he would have to work for two months to pay for it," she told the Guardian. "I said to him, 'Why don't you run away,' and he replied, 'Run away to where? I don't have any money and I can't go home like this.'"

The campaign against slavery has taken centre stage since President Lula da Silva came to power in 2002. In 2003 a national action plan drafted a blacklist of companies involved.

Yet campaigners say a shield of impunity still exists for many slave owners and warn that until a new bill is passed allowing the confiscation of land, little progress will be made.

In the past several politicians have been denounced for exploiting slave labour. Last year, Senator Joao Ribeiro was fined R$760,000 after 38 men, working in slave-like conditions were found on his farm.

Fr Rezende, who fled northern Para state after receiving death threats from pistoleiros (hired guns), claimed that the landowners reasoned the workers were uncivilised, and that they were offering them work and food.

"It is a cultural problem," added Ms Oliveira "The landowners say, 'Everybody does this, so why should I be different?"