"With each new project to tap water somewhere else, demand for water only increases, and at an ever greater cost to China's environment and economy," says the group's report given to reporters on Thursday.

"Whether diverting surface water or digging ever-deeper for groundwater, the underlying solution is like trying to quench thirst by drinking poison."

The Beijing Games have been beset by worries that air pollution will impair athletes. Yet Probe International's study suggests the Games' thirst for cheap and plentiful water will also leave an environmental burden.

Strained underground sources supply over two thirds of Beijing municipality's needs, and since 2004 the city has also begun drawing on "karst" groundwater supplies 1 km (0.62 miles) or deeper below the surface.

These deep underground sources, stored in porous rock, were originally set aside for use only in times of war and emergency, the report says.

Beijing's thirst for water for the Games has also piled pressure on Hebei, the largely rural province next to the capital that supplies much of its water.

To ensure there was no risk of Beijing running short for the Games, officials ordered a 309-km (192-mile) northern section of the larger South-North Water Transfer Project first be completed to pump more water, if needed, from Hebei.

Hebei is already one of the country's most water-short provinces after a decade-long drought, but nonetheless supplies Beijing with about 80 percent of its water.

A visit there this week showed the canal project has been completed, but only barely, and many farmers have been left weighing the costs of giving up land, water and crops for the sake of presenting a verdant Olympic city.

"We've been lucky with the rains this year, but we still don't have enough water," said Liu Xiuge, a middle-aged farmer in Gaochang Village, who said farmers had planted corn, rather than wheat, because not enough water was available.

Other villagers there said wells were running low because engineers had pumped away groundwater to make way for the canal, about 100 metres across, now cutting through the fields.

"If you dig a well now, you hit rock before there's any water. It was never like that before," said Wang Guiju, a 55-year-old grandmother.

"We've had good fortune with the rain this year. But what happens next year when we have another drought? I don't think they'll be rushing to help us."

Such complaints reflect a much broader water crisis facing northern China and the national capital, where industrialisation and population growth have overwhelmed conservation concerns, says the Probe International report, written by Chinese experts who requested anonymity.

"Long distance diversion is extraordinarily expensive and environmentally damaging," says the report, which calls for reforms to water pricing and economic policy so consumers are encouraged to save water.

(Editing by Nick Macfie)