Sao Paulo is thirsty.

A severe drought is hitting Brazil's largest city and thriving economic capital with no end in sight, threatening the municipal water supply to millions of people.

The water at the Cantareira reservoirs, which supply about nine million of the 20 million people in the metropolitan area, is at its lowest level ever amid the region's worst drought in 45 years.

Other reservoirs are also in distress after 15 months of overstretching.

And more dry weather is forecast.

Prosecutors have threatened to sue the state government to make it begin rationing water, warning that Sao Paulo is facing "the worst water crisis ever to hit the region and the collapse of its entire reservoir system."

But with Governor Geraldo Alckmin up for reelection in October, his administration has vowed to handle the crisis without rationing the city's water.

State water company Sabesp has encouraged consumers to reduce their usage but also downplayed the shortage, saying it will transfer water from other dams and use emergency reserves if necessary.

It says it has enough supplies to last until March 2015 and has vowed not to implement rationing.

- Ration by stealth? -

But many Sabesp customers suspect their supplies are already being rationed.

"Last week we went four whole days without water," said Adilson Becerra, a 36-year-old salesman who lives in the southern suburbs.

"From one day to the next we were left without water. Nobody told us anything. From Thursday to Sunday, not a single drop came out of the tap."

In a recent survey by newspaper Folha de Sao Paulo, 46 percent of Sao Paulo residents reported having their water cut at least once so far this month, up from 35 percent in the previous survey in May.

The region's rainy season is the southern hemisphere summer, from October to March. But the period was exceptionally dry in 2013-2014.

And another dry year is forecast for 2014-2015.

To deal with the drought, Sabesp has begun using water from the bottom of its reservoirs -- so-called "dead volume" -- but prosecutors told the company this posed a health risk.

The lack of rain has also hit Brazil's main source of power, hydroelectric dams, forcing officials to turn to more expensive and more polluting thermoelectric plants.

"The situation is critical for both potable water and energy. The ideal thing would be for people to reduce their water consumption," said Cristopher Vlavianos, the president of independent energy distributor Comerc.

- 'Lack of forethought' -

Elsewhere in Sao Paulo state, home to 41.2 million people including the capital, rationing has already begun.

In Guarulhos, a city of 1.3 million people, officials have been implementing water cuts since March.

It has been a shock for many in Brazil, a sprawling country used to abundance.

"We tend to think we'll never lack water in Brazil. In 2009 and 2010, we had an excess of water, but it was lost because we didn't know where to store it. It's a complicated situation," said the director of the Guarulhos water company, Marco Aurelio Cardoso.

The problem is critical for Brazil, the largest economy in Latin America and the world's seventh-largest. Sao Paulo is the country's industrial heart, the capital of the auto, aviation, chemical, construction, finance, mining and oil sectors.

"The lack of water will have an economic impact, though we don't yet know how big," said economist Andre Perfeito of consultancy Gradual Investimentos.

"But the worst impact is on confidence and expectations because this looks like the result of bad administration."

Officials should have seen the problem coming, said geographer Paulo Roberto Moraes of the Catholic University of Sao Paulo.

"For years we were at the limit of our reserves and nothing was done to improve infrastructure," even though the population and demand continued increasing, he said.

"We don't know what will happen if the drought continues. Probably a collapse."