Brazil's largest city, Sao Paulo, is on the verge of rationing water because of a severe drought, according to a nonprofit group that monitors regional water resources.
The Cantareira water system is supplied to more than 10 million in South America's largest city and is more than 75 percent empty.
The city could possibly see shortages when Brazil hosts the World Cup soccer tournament in June and July.
January was the hottest month on record in the city and meteorologists expect little rain or relief in the next week.
An unusually strong high-pressure system over southeast Brazil has blocked the summer rains in recent weeks, causing Sao Paulo's main reservoir to fall to just 20.9 percent of its capacity as of Wednesday, its lowest level in a decade.
Some small cities in Sao Paulo state have already seen water shortages and rationing imposed.
Any decisions about water rationing in the Sao Paulo metropolitan area, which has an estimated population of 20 million, ultimately reside with the state government and Sabesp, the region's water utility.
Sabesp has not responded to numerous requests by Reuters over the past week to discuss the outlook for rationing.
Sao Paulo Governor Geraldo Alckmin said on Tuesday that if rains return to the region by Feb. 15, rationing in the city of Sao Paulo should be avoidable.
But water levels are so low that rationing should have started already, according to the PCJ Consortium, a nonprofit group that monitors rivers feeding into the Cantareira System, as the main reservoir is known. The group is supported by area municipalities and several large companies.
"I would have already shut off the tap" to consumers on a controlled basis, the group's project manager, José Cezar Saad, told Reuters.
Rationing was necessary, Saad said "because in reality, the big problem isn't even today, it's the normal dry season that we're going to face starting in May and June."
A recent report by the group said the possibility of water shortages during the World Cup, which is due to start in Sao Paulo on June 12, is "large."
Fifteen of the 32 international teams that will compete in the tournament have chosen Sao Paulo state as their training base.
Meteorologists and other experts, however, have said that Sao Paulo can still avoid a major water crisis if rains resume in late February or March. Other regions of Brazil, including the other 11 cities that will host World Cup matches, have not experienced such a severe drought.
The last major incidence of water rationing in Sao Paulo was in 2003.
Economists have also worried that water rationing or shortages could take a toll on Brazil's fragile economy, which is expected to grow just 2 percent this year.
(Reporting by Alberto Alerigi and Leonardo Goy, writing by Brian Winter; editing by G Crosse)