A decision by Brazil's Congress to curb the powers of labor auditors threatens to jeopardize an antislavery program that led to the release of more than 15,000 slaves and made Brazil a world leader in fighting indentured servitude, officials and activists said here this week.

Brazilian lawmakers passed a new law on Tuesday that unites two federal tax bodies in a bid to streamline a complex and bureaucratic system.

But one key amendment in the bill strips labor auditors and prosecutors of their power to determine the relationship between employers and employees. The amendment was included at the behest of media companies, who routinely use freelancers and who, under the existing statute, could be punished for doing so by labor auditors.

Under the new law, judges are now responsible for defining that relationship. An apparently unintended consequence of the change is that auditors can no longer determine what constitutes slavery, say prosecutors, auditors, and human rights activists.

"This will make it substantially more difficult to register and document incidents," said Walter Nunes, president of the federal judges association.

The amendment also prevents prosecutors from leveling immediate fines on employers found to have deliberately kept slaves. Until now, prosecutors who accompany government auditors to track down reports of slavery in the remote outposts of the Amazon jungle had the power to fine employers they determined were keeping slaves. Last year guilty employers were forced to pay around $3 million in back pay to workers they used as indentured servants.

Congress's decision was met with anger and disappointment by antislavery campaigners, who say it will neutralize all the previous work done by a government that had made eradicating slavery one of its proudest boasts.

"The campaign against slavery was successful because auditors were able to recognize slavery and demand that employers pay up," said Rosa Jorge, the president of the auditor's union.

"Now, employees have to go through the justice system, and there are two things that can happen," she says. "Either they cut a deal – and normally that is not good for workers – or they spend years waiting for the case to come to court. But these slaves won't go to court, many of them don't even now that courts exist."

Brazil has a hidden but serious problem of slave labor. The International Labor Organization (ILO) estimates that there are 40,000 slaves working in Brazil today.

President Luiz Inacio Lula da Silva made eradicating what he called "Brazil's shame" one of his main goals, and he doubled the Department of Labor Oversight's budget and increased the number of mobile antislavery units from four to seven.

The units follow up on reports of abuse in some of the most inhospitable places in this continent-sized nation and free workers lured into slavery by promises of big payoffs.

Workers are routinely taken to remote outposts and prevented from leaving, sometimes by armed guards. The captives, most of whom are poor and unskilled, are put to work and made to pay exorbitant prices for basics like tools, soap, and food. Many end up owing money, even after spending months doing hard work like cutting down trees or clearing cattle pastures.

Lula's campaign was a huge success, with officials freeing more than 15,000 slaves since he came to power in 2003. The ILO lauded Lula's efforts in a 2005 report on slavery worldwide.

Now, though, that work is in jeopardy, and unionists, lawyers, prosecutors, and human rights workers are banding together to pressure Lula into vetoing the amendment.

Myriad voices have condemned the clause, with judges saying it will add to the backlog in their courts, ministers calling it detrimental to the average worker and the Brazilian Bar Association ruling it unconstitutional.

About the only ones happy with the decision are big businesses. The decision gives employers more leeway to deal with freelance employees, a move that, they argue, is vital in a job market where short-term contracts are increasingly common.

Large companies are now free to hire freelancers without paying them the benefits entitled to full employees. Employers, moreover, will no longer be at the mercy of "the arbitrariness of auditors," says Armando Monteiro, the president of the National Confederation of Industry.