Mexico City has declared an environmental yesterday, ordering vehicles off the road and postponing the semi-finals of the first-division football league as a blanket of smog enveloped the sprawling capital.

A series of wildfires on the city’s outskirts have combined with stagnant weather conditions to cloak the metropolitan area of more than 20 million people in a grey cloud. Pollution levels passed the critical 150 points on the authorities’ air quality index.

The Mexican football league said that under the recommendation of city officials, it had decided to postpone the first leg of the first-division semi-final between Leon and Mexico City club America from today night to tomorrow because of the pollution.