The Lao government last month notified the Mekong River Commission of its decision to push ahead with development of the Don Sahong Hydropower Project in the Siphandone area of Champasak province - two kilometres from the Lao-Cambodian border.
Teerapong Pomun, a member of Thailand's Living Rivers Siam Association, said on Thursday his group and its allies would submit a letter to Prime Minister Yingluck Shinawatra demanding the government raise concerns about the dam construction.
Ms Yingluck should call on the Lao government to follow the "prior consultation process", an MRC regulation which states that any dam project along the Mekong requires feedback from neighbouring countries, he said.
Mr Teerapong said the Lao government claimed the dam would not be situated on the mainstream Mekong River, a claim he refutes.
Vientiane has said the 260-megawatt dam will be situated on the Hou Sahong, which is "one of the braided channels of the Mekong River".
Pianporn Deetes, Thailand campaign coordinator at International Rivers, said the dam site is an important feeding and breeding ground for Mekong fish.
"The dam will deplete food security for the people in the Mekong downstream countries. We have to trade off fertile ecological systems and fish stocks with the dam, which will produce only a small amount of electricity," she said.
International Rivers representative Ame Trandem accused Laos of failing to submit its plans for the dam for consultation and regional decision making by the countries which will feel the impact.
"If the MRC fails to clamp down on Laos, it will be failing its mandate and will lose any validity they have left as an organisation," she said.
Following the construction of the controversial Xayaburi Dam, the Hou Sahong Dam will create a barrier across the only channel that allows for fish migration year-round, according to International Rivers.
"The Don Sahong Dam will only push Cambodia and Vietnam closer to a food crisis," said Chhith Sam Ath of the NGO Forum on Cambodia. "As the project is next to Cambodia's border, have they forgotten that fish are our lifeline and the backbone of our economy?"
The MRC on Thursday issued a statement confirming the Lao government will start building the dam next month.
"Lao PDR submitted the project as an intra-basin water use on the Hou Sahong channel under the process of notification. This will enable the notified member countries to foresee the project's water use and any impact stemming from this," MRC Secretariat CEO Hans Guttman said.
He added Vientiane has indicated its willingness to discuss the project with other MRC member countries.