Mark Kinver at the BBC:
The UN World Water Development Report also blames a lack of resources and environmental changes for the problem.
The study calls for better leadership if a goal of halving the proportion of people without proper access to safe water by 2015 is to be achieved.
The findings will be outlined next week at the World Water Forum in Mexico.
Described as the most comprehensive assessment to date of the world’s freshwater supplies, the report said that politicians, businesses and aid charities all had a role to play in addressing the problem.