Governments, not private firms, must take responsibility for getting water to their people, a new report argues.
Private firms do not invest where it is most needed, WDM argues
"Private companies only invest where they can make a profit, not where there is the greatest need," Peter Hardstaff of the World Development Movement says.
The organisation has launched the report, Pipe Dreams, on the eve of the United Nations' World Water Day.
The Sustainable Development Network, on the other hand, argued last week that free markets improved water services.
The WDM is unconvinced by the conclusions of the Sustainable Development Network, an array of 30 non-governmental organisations.
"Time and again the private sector has failed to deliver the promised investment," Mr Hardstaff, WDM's director of policy, said.
A look at investment around the world showed that private companies did not focus on areas of greatest need, he argued.
"Less than 1% of private company investment has gone to sub-Saharan Africa and South Asia."
The report, written with Public Services International, describes sub-Saharan Africa as "the area that needs investment in water services the most".
But it says the private sector has failed to deliver the level of investment it promised in every single case where it has been responsible for extending access to water in the region.
"The private sector is not sufficiently reliable to manage these essential public services. Government must assume its responsibilities," David Boys of Public Services International said.
The report's authors, David Hall and Emanuele Lobina, are associated with the Public Services International Research Unit at the University of Greenwich, London, UK.
Their findings differ sharply from those of the Sustainable Development Network, which published a book last week urging government deregulation.
The book's editor, Kendra Okonski, said small-scale entrepreneurs were able to meet demands that governments could not.
"That is not to say it is satisfactory, but it is an example of people providing services where governments are not," she said.