Besides environmental concerns over the clearing of forests and jungles to grow biofuel crops, the Church's Pastoral da Terra (CPT) commission highlighted slave labor as a blotch on the biofuel industry.

Nearly 7,000 people were freed from virtual slave labor in Brazil's sugar cane fields from 2003 to 2008, the CPT said in a statement issued at the start of a five-day biofuel conference in Sao Paulo attended by 40 countries.

The CPT said reports of forced labor had "increased dramatically in Brazil's sugar industry, "where the proportion of workers freed from conditions analogous to slavery went from 10 percent of the total workforce in 2003-2006 ... to 51 percent in 2007 ... and 52 percent so far in 2008."

Brazil's deputy Foreign Minister for energy and technology Andre Amado told reporters he was "outraged" that Brazil's biofuel industry was the target of a "denigration" campaign.

But CPT fired back with a statement calling the Foreign Ministry's dismissal of investigative work done by other government officials into forced labor in the country "scandalous."

"You cannot dismiss the fact that sugar cane cutters are transported and housed worse than animals, forced to work round the clock to exhaustion and even to death, all ... in the name of the frantic race for productivity," CPT said.

"Without taking such (human) cost into account, you cannot promote the relative benefits of Brazil's sugar and ethanol on the global market," it added.