Bangkok motorcyclist decapitated by metal floor sheet of trailer truck that he rode into the back of.

A motorcyclist was decapitated when he ran into a metal sheet fitted to the rear of a 22-wheel trailer truck.

It was parked in the left lane of a road in Samut Sakhon early Monday.

Police said the accident happened at 3am on the Bangkok-bound road just before the railway bridge in Tambon Khok Krabue.

The motorcyclist was identified as Yossapat Thongyu, 21.

His body and motorcycle were found at the rear of the truck and his head a further 10 metres away.

The metal sheet was apparently used to extend the floor so that the truck could carry more rocks.

The truck driver, Padungsak Changphet, 27, told police that he had parked the truck in the left lane after offloading the rocks.

He said he turned on emergency lights to warn other motorists.

After hearing a noise at the rear of his truck, he climbed down from the cab and discovered the body.

Police will conduct an investigation before deciding whether to charge the truck driver.