The commission was responding to an appeal lodged at the end of last year by non-government organisations including the Pará Society for the Defence of Human Rights. It asserts that the Brazilian authorities failed to organise proper consultation of the people affected by the dam project. This claim allegedly angered President Dilma Rousseff.

Among the dozen or so tribes concerned, the Arará and Paquiçamba groups would be hardest hit. It is not yet entirely clear how the dam would affect the region's ecosystem. A group of independent Brazilian scientists, who recently studied the environmental impact report, concluded that the project was not viable. The Xingú river basin is home to four times more biodiversity that the whole of Europe. The dam project would lead to the disappearance of some 1,000 reptile, bird and fish species. The dam reservoir would flood about 500 sq km of land, officially entailing the displacement of 16,000 people.

Supporters of the hydroelectric scheme say it is essential to supportBrazil's economic growth. Belo Monte is expected to supply more than a 10th of electricity output by 2019.

Ten court actions have been initiated against the project since 2001, none of which has reached its term. In its letter the commission asked the Brazilian government to "conduct consultation processes, in fulfilment of its international obligations", explaining that this means "prior consultations that are free, informed, of good faith, culturally appropriate, and with the aim of reaching an agreement".

The head of Brazil's electrical energy agency, Nelson Hubner, insisted that "the indigenous communities and other representatives of society" took part in all the meetings. "All the procedures were followed exactly, with the rigour our legislation requires," he added in an interview published by the daily O Globo. The government said work would continue.

In February indigenous group delegates from the Xingú basin handed Rousseff a petition opposing the dam signed by half a million people. But Norte Energía got the go-ahead for preparatory works last month.

**This article originally appeared in Le Monde