Brazil has been accused of turning its back on its duty to protect the Amazon after the resignation of its award-winning Environment Minister fuelled fresh fears over the fate of the forest. The departure of Marina Silva, who admitted she was losing the battle to get green voices heard amidst the rush for economic development, has been greeted with dismay by conservationists.

"She was the environment's guardian angel," said Frank Guggenheim, executive director for Greenpeace in Brazil. "Now Brazil's environment is orphaned."

In a letter to President Luiz Inacio Lula da Silva, Ms Silva said that her efforts to protect the rainforest acknowledged as the "lungs of the planet" were being thwarted by powerful business lobbies. "Your Excellency was a witness to the growing resistance found by our team in important sectors of the government and society," she wrote.

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The decision by Ms Silva to walk away five years on from her triumphant unveiling as a minister in President Lula's first term has underlined just how far the former trade union hero's administration has drifted from the promises made in its green heyday.

"Her resignation is a disaster for the Lula administration," said Jose Maria Cardoso da Silva, of Conservation International. "If the government had any global credibility in environmental issues, it was because of minister Marina."

The Latin American giant's supposed progress on environmental protection has unravelled in the past year as revelations of record levels of deforestation, violent land disputes and runaway forest fires have followed in quick succession. The worldwide boom in agricultural commodities has created an unparalleled thirst for land and energy in Brazil, and the result has been a potentially catastrophic land grab into the world's largest remaining rainforest. The Amazon basin is home to one in 10 of the world's mammals and 15 per cent of its land-based plant species. It holds more than half of the world's fresh water and its vast forests act as the largest carbon sink on the planet, providing a vital check on the greenhouse effect.

Since President Lula won a second term Ms Silva found herself a lone voice in a government acutely aware that its own political future depended on the vast agribusiness interests she was trying to rein in. The final breakdown in her relationship with the President came after he gave the green light to massive road and dam-building projects in the Amazon basin, and a plan she drafted for the sustainable management of the region was taken from her and handed to a business-friendly fellow minister.

Marcelo Furtado, the campaign director for Greenpeace Brazil, said the resignation was "disastrous" and blamed it on the government's Amazon policy and pressure to ease environmental regulations on factories.

"Although Lula has adopted the environmental talk, the practice is development at whatever cost," he said. Next week, the Amazonian city of Alta Mira will host the largest ever gathering of indigenous leaders in a bid to stop a massive hydroelectric dam being built on the Xingu river, a tributary of the Amazon. Although the government claims no decision has been made on the Bel Monte project it's believed to have already committed itself to the construction despite experts warning of potentially dire environmental consequences.

The resignation brings a sad close to Ms Silva's relationship with President Lula, whose personal story closely mirrors her own remarkable journey as the daughter of an impoverished rubber tapper who rose to be a government minister and internationally recognised environmental champion. Ms Silva spent her childhood drawing rubber sap from trees and hunting and fishing to help support her large family in the Amazonian state of Acre. It was only heavy metal poisoning from polluted water and the contraction of tropical diseases that brought her to the city as an illiterate 16-year-old. Working as a maid, she taught herself to read and put herself through university, emerging as a vocal figure in the rubber tappers' union and a close ally to Chico Mendes, the movement's inspirational leader whose brutal murder would cause an international outcry.

Together the pair led a campaign to halt the disastrous deforestation and rampant eviction of forest-dwelling communities to make way for the logging and ranching that still threaten the Amazon.

The tappers' idea of creating sustainable reserves where forest people can make a livelihood from extractive industries has become a global model for managing forests and Acre now has a two-million-hectare reserve.

Health problems Ms Silva inherited from her youth have led to long periods in hospital but in 1994 she became the first rubber tapper elected to Brazil's senate. The winner of inter-national awards, including the Goldman prize, she has also provided credibility on environmental issues for her former boss.

Roberto Smeraldi, head of Friends of the Earth Brazil, said her greatest legacy may be her decision to walk away. "The emperor is naked now: Lula no longer has a smokescreen to show a policy and implement the opposite of it. He will have a problem, since Marina was perfect for him: she accepted anything he imposed and at the same time acted as a green seal for the Brazilian government."

Andrew Mitchell, a leading forests expert and director of the Global Canopy Programme, said: "The Amazon provides the vital rainfall on which Brazil's crops and hydropower depend, as well as regulating the global climate for the rest of us; losing all that is too big a price to pay."

Enemies of the Amazon rainforest

RANCHING

The explosion of cattle ranching exactly mirrors the dramatic increase in deforestation. The world's leading beef exporter has ignored the link and pumped more money into slaughter houses with the help of the World Bank.

MINING

The soaring price of gold and minerals has revived old mines and spurred the creation of hundreds of new ones. Major mining projects not only require large clearings in the forest, they also leave a toxic legacy of pollution.

DAMS AND ROADS

Every study shows that more roads bring more people and destroy more forest. But the cycle continues. There is noevidence that massive hydroelectric dams deliver benefits to communities or cheap electricity.

SOYA

The worldwide boom in agricultural commodities has bitten enormous chunks out of the rainforest. Vast soy plantations supply the demand for livestock feed and bio-fuels, and make a fortune for agribusiness giants.