One of the five speedboat companions of late actress Nida "Tangmo" Patcharaveerapong has admitted to causing the boat to lurch, which threw Nida overboard to her death, police said on Tuesday.

Police said Phaiboon "Robert" Trikanjananun already told investigators that he had little knowledge about driving a boat and wanted to try it on the night of Feb 24. That night, he and five other people, including Nida, were on a speedboat trip in the Chao Phraya River.

As Mr Phaiboon was behind the steering wheel, Nida, 37, fell into the river by a sudden lurch of the boat as she was about to get up from the back of the vessel, police quoted Mr Phaiboon as saying.

Police would have to wait about a week for further forensic examination results to determine the cause of a deep cut on Nida's left thigh. The results may lead to another charge, police added.

The five people on the speedboat also admitted that they were drinking on the boat on Feb 24.

Police said evidence and witnesses pointed to negligence causing death and not murder.