The worst drought to hit Brazil's biggest city in decades may leave residents with water service only two days a week.

Sao Paulo water utility company Sabesp says a five days-off, two days-on system would be a last-ditch effort to prevent the collapse of the Cantareira water system.

The reservoir is the largest of six that provide water to some 6 million of the 20 million people living in the metropolitan area of Sao Paulo city. The utility says Cantareira is now down to 5.1 percent of its capacity of 264 billion gallons (1 trillion liters).

Sabesp official Paulo Massato Yoshimoto said Wednesday that "rationing could happen if rainfall does not increase in the reservoir area soon."