His warning comes as India's population approaches one billion people and its supply of water becomes increasingly contaminated by pesticides, heavy metals and natural pollutants.

Narayanan: Water crisis could dwarf oil crisis

Experts say Delhi could even run out of water within 25 years if strict conservation measures are not brought in soon.

Official figures show that around 90% of India's population has access to drinking water.

But people who work at improving the water supply say only just over half the country can count on its water being safe and constantly available.

'A crisis for the poorest of the poor'

President Narayan's warning came at a conference in Delhi which is looking at ways to revive and update age-old techniques of water harvesting and irrigation in India.

He said: "The shortage of water and its growing pollution has acquired the proportion of a crisis for the people, especially the poorest of the poor.

There is plenty of water - but you just can't drink it

"The health costs of water pollution do not get factored into our economic calculations ... leading experts have warned that the world is heading towards a water shock which may dwarf the oil crisis."

Environmental analysts say there are at least 100,000 Indian villages facing severe water shortages.

"It is immensely ironic that India was doomed to turn into a nation of thirst," said Anil Agarwal, director of the Centre for Science and Environment.

"Rain captured from just 1-2% of its land with simple techniques could provide as much as 100 litres of water per person daily - much more than the 2.5 litres needed."

Mr Agarwal said over-exploitation and contamination of ground and river water had brought the country to the brink of a water crisis.

Instead of continuing as the main provider of water, the government should help efforts at the community and household level to collect and conserve water, he added.

Experts looking to the past for help

India has always been a dry land dependent on annual monsoon rains.

In the past village reservoirs and underground storage tanks ensured a year round supply in arrid parts of the country.

These were maintained as community assets, some are still in use today despite being over a 1,000 years old.

But most fell into disrepair, especially when the British started building big irrigation projects in the last century.

This has continued in independent India and now vast areas of once arrable land are water-logged or highly salined.

Another problem is that the government controls much of India's water and distributes it almost free of charge to farmers and city dwellers.