At a meeting of the Mekong River Commission last week, officials from the four countries through which the lower Mekong flows -- Thailand, Cambodia, Vietnam and Laos -- failed to agree on whether to approve the proposed Xayaburi dam on the river in Laos.
Opponents said the dam would threaten fish habitat and wetlands and possibly displace millions of people.
EarthRights International said the delay was a temporary victory as the Thai government has commitments to buy 95 percent of the power from the project and commercial banks are lining up to fund the project.
Furthermore, the group said, if Laos decides to go ahead with the project, there is nothing to stop them from legally going forward.
Laos, which wants to make money by selling electricity generated by the dam, faces stiff opposition from opponents who have expressed concerns about potential damage to the Mekong Delta.
The World Wildlife Fund in a study published last week said the same amount of electricity could be generated, with less environmental impact, from smaller dams built on tributaries of the Mekong.