High-emission or yellow-labeled vehicles will be banned from Beijing roads from July to September to ensure green Olympics and Paralympics as promised by the authorities, environment officials said on Tuesday.

There are currently more than 3.3 million vehicles in the capital. Yellow labels are displayed on the windscreens of more than 300,000 vehicles that fall short of the Euro I emission standard, the lowest of the European emission standards.

"All yellow-labeled vehicles, most of which are freight trucks, will be banned from roads in Beijing from July 1 to Sept 20," said Du Shaozhong, deputy director of the Beijing municipal environmental protection bureau.

"Limiting high-emission vehicles is the top priority in dealing with pollution," Du said.

While a target of 246 blue-sky days was achieved last year, air pollutants remain a major problem for the city, Du said.

A large number of freight trucks, mostly yellow-labeled ones that run through Beijing at night, have contributed to this problem greatly, Du said.

"There are 20 percent more air pollutants in the evening than during the daytime," he said.

"The period around 9 pm is always the time when the density of pollutants in the air start to rise and remain high until about 4 am."

Yellow-labeled vehicles are only permitted to enter Beijing between 7 pm and 8 am.