A collapsed section of road in Bangkok was blamed on an overloaded lorry suspected of making illegal payments to officials because of a sticker on its windscreen.

The truck apparently exceeded the weight limit but was still able to operate on busy Sukhumvit Road, causing damage to the road surface and resulting in two traffic accidents this week.

Prime Minister Srettha Thavisin ordered an investigation after speculation that a green star-shaped sticker with the letter B printed on it, spotted on the windscreen of the 10-wheel truck, is used by truckers so that police who have accepted bribes let them pass by without inspection despite being overweight.

The heavily loaded 10-wheeler broke a slab placed over the opening to an underground cable trench in Phra Khanong district Wednesday morning.

The incident, which took place in the middle lane near Soi Sukhumvit 64/1, was the second after a previous incident occurred on Ratchaprarop on Tuesday.

On Tuesday an SUV and its driver were stuck in a hole on the road surface that opened up immediately after a truck passed over it.

The Ministry of Transport, Bangkok Metropolitan Administration (BMA) and the Department of Highways will work together to look into the problem of illegally loaded trucks, Mr Srettha said.

The Metropolitan Police Bureau (MPB) has launched a probe into whether the 10-wheeler was to blame on Wednesday if bribes were involved, and the meaning of the suspicious sticker.

Meanwhile the president of the Land Transportation Association of Thailand confirmed the green sticker gives heavy trucks a "free pass" when it comes to exceeding the weight limit and operating in the city outside of the allowed hours.

Without such a sticker, these trucks could never pass through several intersections where the police usually have checkpoints, he said.

He said the green sticker allows trucks to carry dirt to and from construction sites across Bangkok without having to worry about any police interference.

It is given in exchange for bribes to the police and BMA officials, he added.