Prime Minister Prayut Chan-o-cha has told authorities to seize the body of a witness in the "Boss" hit-and-run case who died in an accident last week, and to perform a new autopsy.

Prayut's decision to seize the body of Jaruchart Maadthong was to allow forensic officials to conduct an autopsy as the public questioned the timing of his death.

Jaruchart was killed in a two-motorcycle crash on Huay Kaew Road in Chiang Mai on Thursday. The driver of the other motorcycle, Somchai Tawino, was injured.

Jaruchart and retired Air Marshall Jakkrit Thanomkulabutr were new witnesses brought into the case two years ago.

They testified that Vorayuth Yoovidhya was not driving at high speed and saw the motorcycle of Pol Snr Sgt Maj Wichian Klanprasert cut in front of his Ferrari on Sept 3, 2012.

Their testimony led to the closure of the high-profile case against the Red Bull scion after prosecutors in June dropped the last charge of fatal reckless driving.

Mr Somchai told Phuping Rajavinej police after the accident that he did not know Jaruchart.

But he later said the two first met while drinking at separate tables in a restaurant on the night, and were on their way to find another drinking spot when the accident happened.

Jaruchart lost control of his bike and fell, hitting his head on the road. His motorcycle hit the rear of the bike driven by Mr Somchai, police said after watching CCTV footage of the accident.

Royal Thai Police deputy spokesman Kissana Phathanacharoen said the autopsy will be conducted at Maharaj Nakhon Chiang Mai Hospital of Chiang Mai University.

"The death of Jaruchart is unnatural, and there is public suspicion about it," Pol Col Kissana said.