The number of vehicles on China's roads will more than double to at least 200,000,000 by 2020, said the top official on Monday, further straining the nation's environment and energy supply.

China must make it a top priority to develop fuel-efficient and alternative energy cars. China's auto sales is the biggest in the world, hit to 13.64 million units last year, overtaking the US as the world's top car market, while sales this year are forecast to hit 15 million units.

At the end of 2009, there were 76,200,000 vehicles in China, according to government figures. China's auto sales have slowed in recent months due to seasonal factors, but August saw a surprising 55.7% year on year jump, boosted by Beijing's new subsidies for energy-saving vehicles. Those cars use has brought mounting concerns over pollution, soaring energy demand, and traffic gridlock.

“vehicle exhausts had overtaken other major sources as the top cause of air pollution in cities. Road congestion has also worsened, highlighted by a recurring traffic jam dozens of kilometres (miles) long on a major highway leading into Beijing from the northwest”, said the country's environment minister, Liu Ziquan, on Monday.