The court was on Monday hearing an appeal by seven people convicted with bribing traffic police to let overloaded trucks pass through without paying a fine.

An investigation had found that the gang had hooked up with traffic police and inspectors in 2014 for protection and the racket was carried out in systematic fashion.

It had logos made and sold them to transport operators in Ho Chi Minh City and the nearby industrial provinces of Binh Duong and Dong Nai for a monthly price of VND2.5-3 million ($110-132). The logos were put up on their trucks. When the officers saw them, they let them go despite their excessive loads.

The gang made nearly VND23 billion in around a year before the operation was busted. It claimed to have paid 80 officers VND5 billion ($216,300) to let the trucks with the logos go, but only one of the identified officers admitted to receiving any money.

Nguyen Canh Chan, 46, an officer in Dong Nai Province, admitted to taking VND300 million ($13,000) from the gang and passing on another VND900 million ($39,000) to his superiors. He also said all accused officers did take the bribes and offered to identify them.

But investigators who recommended charges against the gang said there was not enough evidence to prosecute the officers though the court slammed them for not investigating rigorously enough.

The first instance trial in October last year only jailed Chan for brokering bribery and the bribe givers.

The supreme court said: "The investigators did not conduct a thorough probe. The 80 police officers and traffic inspectors in Binh Duong and Dong Nai provinces as well as HCMC were not thoroughly investigated."

The judge pointed out that the indictment admits the leader of the gang, Nguyen Van Thoi, 43, and one of his accomplices, Le Thi Cam Van, 37, could identify the officers who allegedly took the bribes and know their ages, offices and even phone numbers.

"The criminal activity of the defendants has caused the moral erosion of some government officers which has a negative influence on traffic activities... and has generated public outrage."

A fresh investigation and trial will now take place, but the charges against the defendants will remain.

Overloaded trucks are often blamed for road accidents in Vietnam, but many businesses get away by paying bribes to police officers.