Richard Connor, lead author of the report, told a news conference yesterday that water and energy supplies are interdependent because water is required to produce nearly all types of energy.
He warned that growing competition for the resources could lead to legal disputes and social upheavals.
According to the World Water Development Report 2014, the challenge is to provide energy and clean water to all people.
Today, an estimated 768 million people don't have access to clean water, although by some estimates the number could be as high as 3.5 billion, and 2.5 billion don't have access to toilets or latrines, the report said.
"The fact that these figures are often representative of the same people is evidenced by a close association between respiratory diseases caused by indoor air pollution, and diarrhea and related waterborne diseases caused by a lack of safe drinking water and sanitation," it said.
The report said global demand for water is projected to increase by some 55 per cent by 2050, mainly because of growing demand from manufacturing, thermal electricity generation, and domestic use.
"This enormous increase in the demand for energy and electricity in particular will place tremendous pressure on already limited water resources," Connor said, pointing to the diminishing supply of the world's groundwater and the 20 per cent currently being overexploited.
He said "the challenge will be greatest in developing and emerging economies where we see the greatest demand for energy, and where other water-dependent sectors such as agriculture, industry and urban areas are often growing at an unprecedented pace".
