Mr Qin, 68, was among more than 2,600 delegates to the National People's Congress, the country's normally docile rubber-stamp parliament, who were asked to approve the contentious damming of the Yangtze River in 1992.

'A lot of people like me knew very little about the huge hydroelectric power plant before the vote,' he said. 'But we had heard it was a controversial project shrouded by a long list of concerns such as water pollution, silting, funding and resettlement.'

He recalled intensive brainstorming sessions on the dam project by government officials and experts, which focused on the positive effects of flood control, clean power generation and navigation. Despite such heavy lobbying, a third of the delegates refused to endorse construction of the 185-metre-high dam in an unprecedented show of defiance.

'It was fairly normal to see people [at the time] with conflicting views on such a gigantic dam,' he said.

Admitting he still had reservations about the project, the former Guangxi engineer cautioned against the country's current dam-building frenzy in the search for hydroelectric power to feed its energy-hungry economy.

'We must be aware of the adverse impact of hydropower plants, especially those beyond our coping abilities, and avoid making irreversible mistakes by building too many dams on major rivers,' he said.

While it is no secret the project has had its share of harsh criticism and controversy, questioning it in public is still considered off-limits on the mainland, according to analysts.

But the embarrassing silence was broken when several officials involved in the decision-making process and construction of the dam spoke of its adverse impact.

In a bid to distance himself from the project, a former top water- resources official who voted in favour of the dam recently made a rare admission that he had had serious reservations about its feasibility and ecological hazards.

He said that given its huge financial, environmental and social costs, the Three Gorges Dam was apparently the most expensive - yet far from efficient - solution to taming the Yangtze floods. 'For example, it should have been more effective and cheaper to build several smaller dams on the Yangtze's tributaries,' he said.

But the damming of China's longest waterway was finally approved in 1992 for political and arguably practical reasons, he said.

The mainland's highly centralised system meant bigger projects were favoured, while smaller and less expensive solutions often failed to impress politicians craving for greatness, he said.

But he expressed concern over the grave consequences of seasonal fluctuations in the reservoir's water levels and the effect of the faster flow of water discharged from the dam on downstream embankments. He noted that geological disasters, such as landslides, had been on the rise.

His views were backed by a group of experts and officials, including Wang Xiaofeng , director of the State Council's Three Gorges Project Construction Committee Executive Office, who highlighted frequent geological disasters in the reservoir area at a September seminar.

They issued bold warnings that lives of local people had been threatened by environmental problems and the dam could lead to catastrophe in the absence of preventive measures, according to Xinhua.

While Premier Wen Jiabao also reportedly voiced concerns over unforeseen environmental impact, officials in charge of the project have rejected the warnings of daunting challenges and dire consequences.

During a recent foreign-media tour of the dam area, experts from Mr Wang's office and local officials in Chongqing and Hubei , where the reservoir is located, took pains to put a more positive spin on the project.

They stuck to the conclusion of the government-led feasibility study on the dam nearly two decades ago that the 'benefits outweigh negative consequences' and insisted they were prepared to deal with any environmental and geological hazards.

They claimed success in almost every aspect of the dam-building task, ranging from controlling floods and generating electricity to resettling millions of people and combating corruption.

The point-by-point rebuttal of the accusations and criticism by the government even pointed out the dam's ecological benefits in accelerating the long-delayed efforts to reduce pollution and repair landslides.

Liu Di, a senior official from Mr Wang's office, said the huge costs of the dam, including the relocation of roughly 1.4 million people, had paid off with a much improved facility to tame the deadly floods.

But their vigorous defence failed to impress the media.

'We need more real, concrete facts rather than cliched remarks,' said a mainland reporter.

Liu Shukun, a professor of hydraulics at the China Institute of Water Resources and Hydropower Research, said the push for big dams like the Three Gorges has over-emphasised human water needs while ignoring ecosystem conservation.

Citing a government-backed report on the Yangtze, he warned against dozens of hydroelectric dams already built or planned on the upper reaches of the river and its tributaries driven by the demand for energy.

Both Professor Liu and Fan Xiao, a Sichuan-based geologist, challenged the official line that the dam project, built in an area prone to geological hazards, had not exacerbated earthquakes and landslides.

'The rise of water levels in the dam reservoir has made the saturated mountainsides more unstable and triggered additional geological risks, which has left local people more vulnerable to potential disasters,' said Professor Liu.

Beijing's belated campaign to tackle geological hazards did not start until October 2002 amid growing concerns, only a few months before the reservoir was raised to 135 metres above sea level in 2003. At least 12 billion yuan has been spent so far.

'Only a fraction of geological hazards have been dealt with so far,' said Professor Fan. 'Statistics showed small-scale reservoir earthquakes triggered by the rising water level were markedly increasing.

'The delicate ecological balance and geological balances along the Yangtze, shaped through thousands of years, have been destroyed,' he warned.

Weng Lida, former head of the Yangtze River Water Resources Protection Commission, said trees, plants and aquatic organisms had been affected the most.

Chinese sturgeon, one of the oldest species on Earth that has existed for more than 200 million years, is a living witness to the ecological cost of the mainland's modernisation drive.

The number of the endangered migrating fish found in the Yangtze River has dropped sharply from 5,000 in 1981 to about 300 last year.

According to Liu Denghong, , a researcher at a breeding centre for Chinese sturgeon in Yichang city, Hubei, worsening pollution of the river has driven the living fossil to near extinction.

But he played down the impact of the Three Gorges Dam, which has permanently blocked the sturgeon's migration route.

'The dam has affected the natural breeding of fish stocks, but it is not yet devastating,' he said, adding that the government has invested in preserving fish resources and released rare species into the Yangtze every year.