"We're in an emergency situation. The reservoirs which supply Mexico City are at a historic low, at 62 percent capacity when they should be at 85 percent at this time of year," said Jorge Efren Villalon, director general of CONAGUA for the Mexico City valley area.

Supplies will be halved for three days each month from January to May, from seven reservoirs which supply the city of nearly 20 million inhabitants, Villalon said.

"If we don't take preventative measures, reservoir levels will drop to very dangerous levels during the warmer period from March to April," he added.

Some 2.5 million people at the end of distribution networks would be most affected and would be advised to stock up on extra water during the plan, which aims to save some 10 million cubic meters of water, Villalon said.

Mexico City, once a lush area of lakes, is now at risk of running out of clean water as the needs of its growing population outstrip hard-squeezed supplies.