The Chinese capital is one of the most polluted cities in the world and rapidly becoming one of the most congested, with about 1,000 extra cars crowding on to already choked roads every day.

The 300,000 "yellow grade" cars, which fall short of the city's benchmark emissions standards, would be banned from Beijing's roads until Sept. 20, Beijing's transport authority said in a notice posted on its website (www.bjjtgl.gov.cn) on Tuesday.

Authorities had also banned trucks, tractors, and other "low-speed cargo vehicles" from entering Beijing municipal limits from neighbouring provinces, the Beijing Traffic Management Bureau said.

However, the measure appeared to have had little effect on traffic conditions in the city on Tuesday, with major roads seeming as congested as ever.

Beijing claims to have already taken 50 percent of government cars off the roads, and will ban private cars on alternate days from July 20, depending on whether their number plates end in odd or even numbers. The authorities hope such a measure will take 45 percent of the city's 3.29 million cars off the roads and slash car emissions by 63 percent.

The capital has also announced a raft of factory closures and a ban on major construction to take effect for two months from July 20 to improve air quality.

But the country's top environmental chief in May warned that Beijing could only guarantee clean air with favourable weather conditions during the Games, if emissions continued at their current trends. (Reporting by Ian Ransom; Editing by Ben Tan) (For more stories visit our multimedia website "Road to Beijing" at http://www.reuters.com/news/sports/2008olympics; and see our blog at http://blogs.reuters.com/china)