Ecuador has rejected a petition for a referendum on whether the Yasuni National Park in the Amazon should be opened to further oil exploration.
The National Electoral Council said not enough signatures were collected to force a referendum.
Activists from the group Yasunidos, who had gathered the signatures, accused the council of "fraud".
They oppose more oil drilling in the park, saying it would damage one of the world's richest areas of biodiversity.
The electoral authorities validated 359,781 of the 850,000 signatures collected, well under the 583,323 needed by Ecuadorian law.
They said that some of the signatures were repeated up to nine times, or were found to be incomplete or written by children.
Other people, they said, had used fictional names like Bruce Wayne from Batman and Darth Vader from Star Wars.
But Yasunidos, which brings together environmentalists and indigenous groups, claimed the council was biased.
"Almost seven out of 10 signatures were thrown in the bin," it said on Twitter.
"The council talks about irregularities. We talk about fraud."
It has vowed to take the issue to the Inter-American Commission on Human Rights.
Last month, it announced it had collected enough signatures, saying it was "certain" that the referendum would go ahead.
Limited oil exploitation has been taking place in parts of Yasuni, which covers nearly 10,000 sq km (3,860 sq miles), since the 1970s.
But last year President Rafael Correa abandoned a conservation plan that would have seen wealthy nations pay Ecuador not to drill in previously untouched parts of the park.
Mr Correa said the initiative had attracted only a fraction of the $3.6bn (£2.1bn) it had aimed to raise, leaving Ecuador with no choice but to go ahead with drilling. Oil is the country's main export.
He has promised that any earnings from oil drilling would be used for poverty alleviation.
The park supports a huge variety of wildlife, including unique species of birds, monkeys and amphibians.
It is also home to the Huaorani and other indigenous people who had virtually no contact with the outside world until recent decades.
Yasuni oilfields hold an estimated 846 million barrels of crude, 20% of Ecuador's reserves.