Chronic water shortages already affect 40 per cent of the world's population in 80 countries and the situation is set to become far worse with an expanding population and ever-increasing demand for water.
Whereas the main constraint on food production in the developing world has in the past been a shortage of land, it will be a scarcity of water in the future, according to a study by the World Bank.
Global demand for water is increasing at a rate of 2.3 per cent a year, doubling every 21 years. ''Supply can no longer keep up with this rate of growth,'' the World Bank warns.
A billion people in the world today do not have access to clean drinking water and almost as many again lack adequate sanitation facilities. Dirty water causes 80 per cent of the disease in developing countries, killing 10 million people each year.
''A lack of clean water and proper sanitation also can trigger economic disaster,'' the World Bank says. A recent cholera epidemic in Peru, for instance, resulted in loss of agricultural exports and tourism estimated at $1bn (pounds 625m)''or more than three times the amount invested in water supply and sanitation services in Peru during the 1980s''.
Ismail Serageldin, a World Bank official, warned: ''Many of the wars this century were about oil, but wars of the next century will be over water.''
Stephen Lintner, an environmental specialist at the World Bank, said that the Middle East is the most likely focal point for future wars over water. "There is a high potential for conflict of a military nature in the river basins of the Jordan, Tigres and Euphrates,'' he said.
The gradual nature of the crisis requires ''sustained action'', Dr Lintner said. ''We need to treat water as a valuable resource that should be wisely used. Much of the scarcity is not to do with its unavailability, it's because it's badly managed.''
Major trends that are creating a global water emergency are:
t A population explosion, from 5.6bn to about 8bn by 2025.
t Increasing levels of pollution that contaminate clean water supplies, effectively decreasing the amount of water available.
t New sources of water can cost between two and three times that of sources already tapped.
People living in the Middle East and North Africa will experience one of the most precarious water supplies in the world. The growing population in this region has already begun to overwhelm traditional water sources, the World Bank says.
''From 1960 to 2025, per capita renewable water supplies [in the Middle East and North Africa] will have fallen from 3,430 cubic metres to 667 cubic metres, or an 80 per cent drop within a single lifetime.''
The contamination of water supplies by increasing levels of pollution will result in a spiral of decline for water resources. About 95 per cent of the world's sewage is poured straight into rivers, where it is joined by growing amounts of industrial waste.
Mr Serageldin said: ''Most rivers in and around cities and towns in these countries are a little more than open, stinking sewers constitute a reservoir for cholera and other water-related diseases.''