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BEIJING, April 23 (Reuters) - A bus plunged 20 metres (65 feet) off a highway overpass in southwest China on Monday, killing at least 26 people, with overloading and slippery roads to blame, state media said.

It was the most serious accident in China this year and the second serious traffic accident in Chongqing, a mountainous metropolis of 30 million people, in 10 days.

"While police are continuing their investigation they say overnight rain may have caused the road to become slippery, causing the driver to lose control," the official Xinhua news agency said on a day China launched a week-long road safety campaign.

There were 32 people on board, including the driver, when the bus was only licensed to carry 25, Xinhua said. It also said part of the overpass fence support was lacking adequate reinforcement.

A pile-up of 52 vehicles, mostly trucks, killed five people and injured 36 on a Chongqing highway on April 13.

A bus plunged off a bridge in Chongqing last October, killing 30 people and injuring more than 20.

China has the world's deadliest roads, due to popular disregard for traffic regulations, lax police enforcement of laws and a cavalier attitude towards safety.

Last year almost 90,000 people died in traffic accidents and 400,000 were injured -- which works out at around one death every five minutes.

"The burden of injury is falling disproportionately on the young. Lack of awareness and experience means young people are more likely to engage in dangerous behaviour such as speeding, driving without a helmet or seatbelt or drink driving," said Henk Bekedam, the World Health Organisation's representative in China.

"China needs to raise awareness and enforce penalties for people who speed, break road rules, drink drive, or drive without seatbelts," he told the China launch of the U.N. sponsored Global Road Safety Week. (Additional reporting by Vivi Lin)

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