Police will summon the Bentley driver involved in an expressway crash that left eight people injured last week to hear an additional charge after narcotic substances were found in his blood.

Suthat Siwapiromrat will be charged after a blood test at Police General Hospital found narcotic substances in his system.

Police said on Friday that an initial test showed a substance that could be methamphetamine, ketamine, diazepam or nordiazepam. More tests were being done to find exactly what it was.

Early last Sunday morning, the Bentley driven by Mr Suthat rammed into the back of a brand-new Mitsubishi Pajero in the middle lane of the Chaloem Maha Nakhon expressway. The impact caused the Pajero to overturn before it was hit by a fire truck in the right-side lane.

The crash left six people in the Pajero — one of them a four-year-old child — injured, along with two firefighters in the truck.

Mr Suthat exited his Bentley, walked down the expressway and hailed a cab in an apparent bid to flee the scene, according to a report from a team of first responders.

According to one of the first responders, Mr Suthat appeared intoxicated. However, he refused to take a breathalyser test, claiming he had chest pains due to the airbag inflating at the time of the accident. He agreed to take a blood test instead.

The police handling of the case has drawn criticism that the driver was being favoured because of his wealth and connections. A director of several companies and a well-known political donor, Mr Suthat is a brother of New Economics Party leader Manoon Siwapiromrat.

Police will question the police officers who did not conduct an alcohol test on the driver immediately after the crash.