The environmental authorities in Mexico City will keep 40 percent of cars off the roads on Wednesday because of extremely high pollution levels, officials said.

"The Greater Capital Area Environmental Commission (CAME) is activating extraordinary measures due to high ozone levels in the metropolitan area of ​​the Valley of Mexico," CAME said in a statement together with city, regional and federal authorities.

Some 5.5 million vehicles are estimated to circulate in Mexico City's metropolitan area and are considered among the main source of contaminants in the highly polluted city.

Normally, 20 percent of cars are kept off the streets to reduce pollution levels.

But the authorities declared the more serious pollution alert, and ordered double the normal car restriction, after smog in the capital pushed into the air quality index's more worrisome "environmental contingency" zone.

Recent pollution alerts mark a reversal from years of progress to improve air quality after the United Nations declared the Mexican capital the world's most polluted city in the 1990s.