Brazilian President Dilma Rousseff came under enormous pressure Thursday from environmentalists to veto a new forestry bill they fear will speed up deforestation of the Amazon rainforest.

Brazil's powerful agribusiness sector scored a major victory with congressional approval Wednesday of the forestry code reforms, which Rousseff repeatedly promised to veto while on the campaign trail in 2010.

The current code, which dates back to 1965 and which farmers argue is not respected anyway, limits the use of land for farming and mandates that up to 80 percent of privately-owned land in the Amazon rainforest remains intact.

The new bill would allow landowners to cultivate riverbanks and hillsides that were previously exempt, and would provide an amnesty from fines for illegally clearing trees before July 2008.

Farmers, whose industry represent more than five percent of Brazil's GDP, argue that the existing legislation is confused, putting economic development at risk and costing valuable investment.

They say the new code would promote sustainable food production and bring an end to severe environmental restrictions that have forced many smaller farmers off their land.

Brazil's Chamber of Deputies approved the controversial legislation in a 247-184 vote on Wednesday night. The text now goes to Rousseff for ratification after having been approved by the Senate in December.

Paulo Moutinho of the Amazon Environmental Research Institute (IPAM) warned that if Rousseff did not use her veto, years of successful efforts to rein in the ruination of the Amazon would be jeopardized.

"Without a veto by President Dilma Rousseff, Brazil will lose the gains of the last few years which led the country to curb deforestation. We will lose leadership and credibility," Moutinho said.

Opponents say the bill will mean more deforestation and warn it will embarrass the country ahead of hosting the Rio+20 in June, a UN gathering aimed at addressing global threats to the environment.

"It grants amnesty to loggers and raises the risk of environmental disasters in major cities," opposition lawmaker Ricardo Tripoli said as he left Wednesday night's vote. "Now it is important that the president veto it."

Gilberto Carvalho, secretary-general for the presidency, said Rousseff would weigh the decision "with a lot of serenity, without animosity," adding: "We have a great responsibility toward the country."

A recent study by the University of Brasilia found that the new forestry code would increase deforestation in Brazil by 47 percent by 2020.

Carlos Rittl, a WWF climate expert, called it the "biggest environmental retreat in Brazil in decades," while former environment minister Marina Silva urged the public to join a "VetoDilma" online campaign.

But Assuero Doca Veronez, president of the national environmental commission of the National Farming Confederation, said the present code "has long been incompatible with the changes in Brazilian agribusiness."

The proposed reform threatens 690,000 square kilometers (270,000 square miles) of land and would prevent Brazil from reaching its goal of reducing deforestation by 80 percent, according to the Climate Observatory, a network of 26 non-governmental organizations set up in 2002.

Authorities say key reasons for the deforestation of the world's largest rainforest -- a region of amazing biodiversity that is considered crucial to the fight against climate change -- are fires, the advance of agriculture and stockbreeding, and illegal trafficking in timber and minerals.

Deforestation has slowed since Brazil declared war on the practice in 2004, vowing to cut it by 80 percent by 2020.

Between 1996 and 2005, 19,500 square kilometers (7,530 square miles) of forest was cut down on average, peaking in 2004 when more than 27,000 square kilometers was lost.

Better law enforcement and the use of satellite imaging saw the lowest rate of deforestation in 2011 since records began three decades ago. Just over 6,200 square kilometers was cut, a 78 percent reduction on 2004.