By Mark Kinver
BBC News science and nature reporter
Almost 20% of the world's population still lacks access to safe drinking water because of failed policies, an influential report has concluded.
Water-borne diseases killed more than three million people in 2002
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.
Although steady progress had been made in recent years, it said that more still needed to be done.
Unesco, which headed the group of 24 UN agencies that compiled the data, said the report highlighted the need for stronger leadership and co-ordination.
"Good governance is essential for managing our increasingly stretched supplies of freshwater, and indispensable for tackling poverty," says Unesco director-general Koichiro Matsuura.
"There is no one blueprint... but we know it must include adequate institutions - nationally, regionally and locally - strong effective legal frameworks and sufficient human and financial resources."
Unless there was a marked improvement, the report warned, regions such as sub-Saharan Africa would not meet the UN Millennium Development Goal of halving the proportion of people without access to safe drinking water between 1990 and 2015.
Only 12% of nations had managed to meet a deadline to introduce an effective water strategy by 2005.
WORLD WATER FACTS
One billion people without access to clean drinking water
2.6 billion without adequate sanitation
Rapid urbanisation increasing pressure on water resources
30-40% of water 'lost' through illegal tapping and leaks
(Source: UN World Water Report)
Changes to the global climate were also having an impact. The report found that many regions' river and groundwater levels were falling because of lower rainfall and higher evaporation rates.
Rapid urbanisation in a number of developing nations was also affecting people's ability to access water, the data showed.
Governments and local authorities were unable to expand networks quickly enough to keep pace with the number of people moving into towns and cities.
Urbanisation also led to an increase in the size of a number of nations' "water footprints", because of growing consumption in food production, industry and energy generation.
The authors said failure to provide adequate supplies and sanitation was directly linked to poor health and low quality of life among the urban poor, which could act as a trigger for social unrest and conflicts.
Barun Mitra, director of Delhi-based think-tank the Liberty Institute, said the report showed a "bottom-up" approach was needed.
"It is the inability to learn from people on the ground that is at the root of this problem," he told the BBC News website.
"In large countries like India, it is very easy to look at things from the top and say 'we are going to make this reform, or that reform', without recognising the needs of specific people in specific areas."
There were many examples of people in the slum areas on the outskirts of Delhi helping themselves when it came to providing reliable water supplies, Mr Mitra said.
"Nobody wants to wait around for somebody else to solve their problems, because water is a prime necessity. Where governments are failing is by not recognising these informal initiatives and putting them into a formal framework."
'Ignore at our peril'
The report - called Water, A Shared Responsibility - also found that:
- Water quality is declining in most regions, affecting the diversity of freshwater species and ecosystems
- Poor water quality is a key cause of poverty. Around 3.1m people died in 2002 as a result of diarrhoeal diseases and malaria, 90% of whom were children
- The world will require 55% more food by 2030, increasing the demand for irrigation which already accounts for 70% of all freshwater used by humans
- Many places are "losing" 30-40% of water through leakage and illegal extraction
- Political corruption is estimated to cost the water sector millions of dollars every year and undermines services
Carlos Fernandez-Jauregui, deputy co-ordinator of the UN's World Water Assessment Programme, told this website that he hoped the publication's findings would push governments and organisations into action.
"If we continue business as usual the water crisis will get worse - not only in developing countries but also in developed countries," he said. "We ignore this at our peril."
The report's findings will be discussed at the 4th World Water Forum, which begins in Mexico City on 16 March.