It was the second Citybus double-decker to go up in flames in three weeks.

Nobody was injured in either of the incidents, which the Fire Services Department said were not suspicious.

Just two hours before the incident, a truck parked in a car park in Cheung Sha Wan burst into flames when the driver was unloading equipment. The truck was severely damaged but no one was injured.

At 9.35am, another truck caught fire when it reached the West Kowloon Highway near Yau Ma Tei. The fire was quickly put out, again with no injuries.

The Shenzhen bridge incident happened around 12.30pm as the bus on route B3A was carrying more than 30 passengers from Tuen Mun to the border.

When it reached the Shenzhen end of the bridge, the 30-year-old driver, named Chau, found that the left back tire had punctured and arranged for the passengers to continue their trip on another bus.

But around 12.50pm, Chau noticed the tire was on fire and it spread rapidly through the bus. Firefighters put out the fire in 20 minutes but the bus was gutted.

Traffic toward Shenzhen Bay was paralyzed, with a four-kilometer tailback.

A Citybus spokeswoman said the bus had undergone a regular check on December 26 and passed.

On December 26, a Citybus single-decker on route 606 from Siu Sai Wan toward Choi Hung caught fire in Shau Kei Wan.

The driver was unable to contain the blaze with a fire extinguisher and had to call for help.

Hong Kong Polytechnic University department of mechanical engineering professor Lo Kwok-keung said if there was a problem with the back tire, it could have had a dragging effect on the drum brakes, with friction causing the fire.