<b>Richard McGregorin Beijing and Fiona Harvey in London</B>

The recent removal of up to 1.3m cars from Beijing's roads in a test run of measures to ensure clean air at the Olympics Games next year has been declared a success.

City officials said pollutants from vehicles dropped by up to 20 per cent. The number of cars on the road was cut by about a third over four days, starting on Friday, by designating alternate dates on which vehicles with odd and even number plates could be driven.

"The results indicate the measures we took during the four days had a good impact," said Du Shaozhong, a spokesman for the city environmental bureau.

"The tests have made it very clear that we are capable of providing good air quality by the time of the 2008 Olympic Games."

The results announced by the government were at odds with the appearance of the air in Beijing during the test period, which was thick and hazy. The horizon was shrouded in smog, as it of-ten is in the Chinese capital.

According to the benchmark used by the city, the air quality rated a "two" on a scale of one to five, with five being the worst quality.

The still and humid conditions over the test period made "emission diffusion" difficult, officials said, and without the removal of the cars the air would have been worse.

Air pollution was a significant issue in the run-up to the Los Angeles, Atlanta and Seoul games, and the International Olympic Committee has said events could be delayed if the pollution in Beijing is too severe.

Although Beijing's surging car ownership, now expanding at a rate of about 1,000 a day, is often blamed for the city's poor air, there are other factors at play.

Capital Iron & Steel, for many years both the city's largest industrial complex and its biggest producer of pollution, has been mostly closed or shifted into neighbouring Hebei province. But, with a southerly wind, Hebei can contribute up to 70 per cent of particulate matter in Beijing's air, according to a recent paper in the publication Atmospheric Environment.

Industry, cars and a number of new coal-fired power stations coming on stream in surrounding cities and provinces are all big contributors to the capital's air pollution.

"Controlling only local sources in Beijing will not be sufficient to attain the air quality goal set for the Beijing Olympics," the Atmospheric Environment paper said.

Among the many ideas under discussion to keep the air clear is a plan to have part of the electricity grid use wind-generated power from nearby Inner Mongolia, while shutting down some coal-fired stations.

This might help clear the air and give the games an environmental lustre, a theme organisers are keen to promote.

Beijing's experiment with number plates follows several other attempts in cities around the world to cut pollution from vehicles and other sources, which have had mixed results.

Athens pioneered the scheme allowing cars with odd and even numbered registration plates into the city on odd and even numbered days of the month. However, an unintended side-effect of the scheme was that some residents acquired a second car to get around the restrictions. A similar scheme in Mexico City met the same response.

In London, a congestion charge was introduced requiring drivers to pay for entering the centre of the city but, after an initial drop in the number of vehicles, the city has seen drivers returning to their cars.

The grounding of all aircraft in the US in the wake of the September 11 attacks in 2001 inadvertently produced a reduction in air pollution. The clearer skies allowed scientists to gauge the extent of air pollution generated by aircraft, which had previously been difficult to measure