The government of Brazil has abolished a vast national reserve bigger than the size of Denmark to open the area to commercial mineral exploration, drawing sharp criticism from environmentalists and political opponents.

The area, which straddles the northern states of Amapa and Para, is thought to contain rich deposits of gold, iron, manganese and other minerals.

A decree from President Michel Temer published in the official government gazette on Wednesday dissolved the protected area, known as the National Reserve of Copper and Associates (Renca).

Established in 1984 under the then military dictatorship, the protected status of the reserve, which covered roughly 4.6m hectares (17,800 square miles), restricted mining activities to state companies.

Temer has been seeking to stimulate economic activity as Latin America's top economy emerges from the worst economic crisis in more than a century.

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Wednesday's decree stressed that it does not override other existing environmental protection laws, such as protections for native vegetation and nature conservation areas.

But campaign groups such as the World Wildlife Fund (WWF) have expressed concern about the environmental threat to the reserve from potential mining projects.

Brazilian newspaper O Globo quoted Michel de Souza, the public policy coordinator of WWF-Brazil, as saying that the decree is a "catastrophe".

"The Amazon Forest is our biggest asset. In this moment of despair and crisis, they are putting at risk the protected areas that are within the reserve," de Souza said.

Moira Birss, from the nonprofit organisation Amazon Watch, told Al Jazeera that the decision by the Temer administration is "concerning".

"The Amazon is extremely crucial to the global climate," Birss said. "Deforestation and destruction of even small parts of the Amazon have a major ripple effect for the entire global climate.

"It's a really short-sighted move to try to strip away those [environmental and indigenous] protections to what would amount to destroying many resources that in many cases will be likely to benefit foreign companies," she added.

Last month, the executive director of WWF-Brazil Mauricio Voivodic warned of the damages that mining would bring to the reserve.

"Despite the strong economic appeal, the development of mining activity may bring undesirable impacts to the Renca protected areas, such as deforestation, threats to indigenous people and populations, water resource loss, loss of biodiversity, and aggravation of land conflicts," Voivodic said.

More than two-thirds of the Renca area that lies in Amapa state are subject to conservation controls or protections for indigenous areas that would limit mining, leaving only 31 percent open to research and exploration after the area's abolition, according to a 2010 government report.

In April, a report by the mining ministry said that lifting the protected status could provide "access to minerals potentially existing in the region" by letting private companies operate there.

However, some politicians criticised the decree as an act of devastation to the Amazon.

According to O Globo newspaper, opposition Senator Randolfe Rodrigues decried the move as "the biggest attack on the Amazon of the last 50 years".