Former USSR president Mikhail Gorbachev has told the Third World Water Forum in Kyoto that a failure to reverse the global water crisis could lead to "real conflicts" in the future.
By Ben Sutherland
BBC News Online in Kyoto
Mr Gorbachev, who is now president of the International Green Cross, said that there were likely to be severe problems as the demands on water increased together with the planet's population.
Mikhail Gorbachev (l) meets Japanese Prime Minister Junichiro Koizumi (r)
It is estimated that by 2025, two thirds of the world's people will be living in areas of acute water stress.
"If current trends continue, we could be faced with a very grave situation," Mr Gorbachev warned.
It is feared conflicts could arise in areas where rivers and river basins cross state borders.
If a country near a river's source begins using more water, this lowers the amount that reaches countries further downstream.
For example, there is currently concern about what effect a proposed scheme in India to divert the Ganges to currently dry areas might have on the water supply downstream in Bangladesh.
Mr Gorbachev said all countries in river basins would have to co-operate to prevent tensions.
"Water management can only be effective based on the basin approach," he said. "All countries - the entire basin has to be considered together. Otherwise, the dominant countries could control [the water]."
And he stressed that there must be an improvement on the current situation.
"A great majority of countries have not reaffirmed their commitment to
co-operate on water resources. We are facing some real conflicts."
Many multi-governmental committees in river basins already exist, but countries are often reluctant to share information.
This has often been the case in South East Asia with China and the Mekong basin, for example.
China only attends meetings of the Mekong basin countries as an observer, and even that development has occurred only recently.
Mr Gorbachev said legal powers must be made tougher to forestall any potential flashpoints.
"International law should become a more effective instrument," he stated. "Any infringement of water resources is inadmissible."