China's environment ministry has dealt a rare blow to the country's ambitious dam builders by blocking the construction of a controversial hydropower dam that would have disturbed one of the last remaining habitats for some endangered river fish.

The proposed Xiaonanhai dam, upriver from the large municipality of Chongqing in southwest China, has been a rallying point for environmentalists ever since a disgraced former party chief redrew the boundaries of a protected nature preserve to make way for the project.

Environmentalists contended that the dam would have flooded a free-flowing section of the Yangtze river's lower reaches, damaging an important spawning ground for many fish.

But a victory seemed unlikely as they were pitted against China's powerful dam-building lobby. The country is under pressure to reduce smog from coal-fired power plants and is seeking cleaner ways of producing electricity.

The planned Xiaonanhai hydropelectric plant - 700 kilometers upstream from the landmark Three Gorges Dam - would have generated 1,680 megawatts of electricity, according to Chinese media.

Inconspicuously tucked away in approval documents for another dam, the written response from the Ministry of Environmental Protection to the Chinese hydropower firm Three Gorges Corp said the dam would have crossed an "ecological red line" and done irreversible damage to the river's biological diversity.

"In the last 10 years, two investigations have been carried out into construction in precious and unique national protection zones for fish in the lower reaches of the Jinsha river, and the structure and function of the zones have already been heavily impacted," the ministry said in its letter to Three Gorges.

"Your company as well as other units cannot plan or build the Xiaonanhai hydropower plant," it added.

cjc/hg (dpa, Reuters)