Engineering Institute of Thailand secretary-general Amorn Pimanmas said engineers and officials would check which metal beams must be replaced. He said parts would be obtained from the now-demolished Ratchayothin flyover.

The incident led to traffic chaos around Rama IV and nearby streets, with roads closed off as firefighters took 45 minutes to bring the blaze under control before authorities moved in to inspect the scene.

It is suspected that a homeless person or a passing motorist may have discarded a lit cigarette butt at bins stored there by Bangkok Metropolitan Administration (BMA) officials before the fire broke out around 7am. Police were checking CCTV footage of the area.

The blaze, which destroyed around 100 bins, caused heat damage to the bridge’s structure, resulting in cracks and subsidence in the road surface.

The district office reportedly kept over 1,000 new 100-litre garbage bins inside a storage cage under the bridge.

Pathumwan District Office director Morakot Sanitthangkoon said city workers claimed that a group of about five homeless people were spotted sleeping inside the bin-storing room early in the morning and, on seeing the workers, fled through a large hole that had been cut on one side of the cage.

BMA Construction and Maintenance Office director Thiti Songcharoenkit said metal beams supporting outbound lanes on the bridge were bent by heat, while the inbound lane was slightly damaged.

As inquiries into the blaze continued, city police chief Pol Lt-General Sanit Mahathaworn said police would implement measures to alleviate traffic congestion in the area.

The 390-metre, four-lane bridge was once part of a removable iron bridge called the Leopold II viaduct that Belgium built for the 1985 Expo. In 1986, as Thailand suffered severe traffic jams, Belgium offered to donate the metal bridge’s parts so it could be reassembled and reinforced for use in Bangkok.

Deputy Bangkok Governor Jakkapan Pewngarm who led officials to inspect the fire scene yesterday said that he had instructed city officials to remove all items that were stored underneath bridge across the city as to prevent a reoccurance.