The United Nations' wish to halve the number of deaths from people having no access to clean water by 2015 is massively unrealistic, a new report is to say.
Shanty towns are riddled with water-borne diseases
The emphasis of the UN's "Millennium Goal" is to improve the quality of water in rural areas of the world.
But David Satterthwaite, of the International Institute of Environment and Development in London, told BBC World Service's One Planet programme that at least as much effort should be put into towns and cities.
His report to the UN will say that the problem is far worse in urban areas than official figures show.
"According to most official government statistics, most urban people have good water and sanitation," Mr Satterthwaite said.
"Our puzzle has long been that pretty much every city and small urban centre I work with in Africa and Asia, and most in Latin America, has very poor provision, especially for low-income groups."
He added that the problem was particularly bad in "informal settlements" - such as shanty towns - where the very poorest live.
"There are no sewers, few open drains, and most people rely on standpipes," Mr Satterthwaite said.
"It's common for there to be 1,000 people to each stand pipe."
He said the UN's figures needed to be overhauled.
"We reckon you have to multiply four or five times the number of people lacking in good quality sanitation according to the official statistics," he said.
The UN's emphasis is on water supplies in rural areas
And Mr Satterthwaite emphasised that a lack of toilets was having an important impact on health in urban regions.
"Possibly the most shocking thing we found was the number of urban dwellers who rely on open defecation," he said.
"They have no toilet. What we found was, in many cities, there's even a popular term given to open defecation.
"In many cities in Africa, it's known as "flying toilets", because you defecate in a plastic bag and then you throw it."
He said the purpose of the report was to highlight that the problems related to insufficient access to water and the necessary infrastructure were far greater than the UN has allowed for.
"If we were to combine the health impacts of lousy water, lousy sanitation and lousy drainage, it's much bigger than just for water alone."