The Brazilian Chamber of Deputies has approved controversial legislation that eases rules on how much land farmers must preserve as forest.
Brazil's powerful farmers' lobby argues that the changes will promote sustainable food production.
But environmentalists say the new forest code will be a disaster and lead to further destruction of the Amazon.
The bill now goes to President Dilma Rousseff, who may use her veto to remove some clauses.
Wednesday's 247-184 vote in favour of the new forest code capped a year of political wrangling.
Brazil's farmers have long pushed for changes, arguing that uncertainty over the current legislation has undermined investment in the agriculture sector, which accounts for more than 5% of GDP.
Severe environmental restrictions have also forced many smaller farmers off their land, they argue.
Rural producers would have "more stability and political support," said Deputy Paulo Piau, who drew up the Chamber's version of the bill.
"Production and the environment will only benefit from that. With a confused law there is no benefit," he said.
But opponents said the new law was a step back.
"Over the years, we have slowed deforestation and intensified production. Now we are going to modify all the things that resulted in the decrease of deforestation by changing the legislation," said Deputy Sarney Filho.
Greenpeace urged President Rousseff to veto the changes, saying: "It is unbelievable that the forest code is being eroded weeks before Brazil hosts the Rio summit (on sustainable development)."
Several former environment ministers had warned that Brazil would miss its emissions targets if the code were weakened, Greenpeace noted.
Deforestation of the Amazon has slowed in recent years, as a result of better law enforcement, with authorities using satellite images to track clearance.
Under the Forest Code, which dates back to 1965, landowners must conserve a percentage of their terrain forested, ranging from 20% in some regions to 80% in the Amazon.
This provision remains, but environmentalists say other changes to the code will erode key protections.
Under the new bill, farmers will be able to cultivate land closer to hilltops and riverbanks, which are especially vulnerable to erosion if trees are chopped down.
The bill also provides an amnesty from fines for illegally clearing trees before July 2008, although larger landholders would have to replant most of the cleared area or preserve the same amount of land elsewhere.
President Rousseff faces a political dilemma, correspondents say, as she seeks to combine support for economic development, but also uphold environmental pledges made during her election campaign in 2010.