Pollution and overuse of hidden underground water supplies threaten global health and development, the United Nations said on Wednesday.
The United Nations Environment Programme (UNEP) said governments often had little understanding of the social, economic and health risks of sucking their groundwater dry.
"Across the globe, groundwater is being depleted by the demands of megacities and agriculture, while fertiliser run-off and pollution are threatening water quality and public health," UNEP Executive Director Klaus Toepfer wrote in the foreword to the report, issued on the eve of World Environment Day.
Water is the theme of this year's Environment Day, under the U.N. slogan "Water: Two Billion People Are Dying for It!". The United Nations says 80 percent of all deaths from illness in the developing world are caused by lack of access to safe water.
Britain's Environment Agency head Barbara Young, who unveiled the report in London on Wednesday, said governments that keep an eye on rivers and lakes need to pay more attention to the "invisible resource" of water under the ground.
"This isn't just something that's kind of down there and out of sight. This is something that can come up and bite them on the bum," she told reporters.
Depletion of groundwater is a problem in rich countries as well as poor ones. Water levels have been falling by about a metre a year in the arid southern U.S. state of Arizona, where aquifers are depleted about twice as fast as water is replaced.
But the effects are worst felt in poor countries, where there are few other options when village wells run dry.
"It tends to hit small farmers before it hits big farmers or municipal water supplies," said John Chilton, Principal Hydrogeologist at the British Geological Survey, which drew up the report on behalf of UNEP.
"If the well goes dry two years in three instead of one year in five -- because water is the primary resource of farmers they know that resource well."