They say there are flaws in an environmental impact study presented to the Chilean government by Noranda.

"The abstract of the environmental impact study mentions neither the impact the project would have on the environment nor the measures that would be taken to mitigate that impact. This is illegal," Lorenzo Soto, a lawyer representing the campaigners, told Reuters.

Under Chilean law, companies proposing construction projects must compile a study of the effects on the environment that construction could have. This has to be approved by environmental authorities.

If authorities agree that there are omissions in the abstract, this could delay evaluation of the impact study.

The campaigners say the $2.75 billion Alumysa plant, which would have an annual capacity of 440,000 tonnes, would cause irreversible damage to an area of natural beauty as well as have a negative social and economic impact on the region.

Noranda says the plant would not pollute the area and that the impact study complies with the norms.

"The study meets Chilean and international standards and those of the World Bank," project manager Robert Biehl told Reuters.

The environmentalists say the construction of infrastructure such as the dams will involve flooding 9,598 hectares of land inhabited by endangered species of plants and animals.

The plant should take five years to build and be operative for 50 years. The project will involve building three hydroelectric dams, port facilities, 79 km (50 miles) of transmission lines and new and upgraded roads.

Aysen, dotted with volcanoes, glaciers and Pacific fjords, has a lot of potential for adventure and ecological tourism. It is also one of the poorest areas of the country.

"The unique natural qualities of Aysen, its pristine, pollution free environment, could be seriously threatened by the construction of this megaproject," said Peter Hartmann, local director of environmental pressure group Codeff, which has been campaigning for years against the project.

The environmentalists say the study does not provide sufficient detail on the treatment of liquid and solid waste from the plant and prevention of air emissions and leakages.