Swedish journalist Kim Wall died when she was accidentally hit by a heavy hatch cover on board a home-made submarine, the Danish owner of the vessel has testified in court.

Peter Madsen, 46, is charged with killing Wall on his UC3 Nautilus submarine and mutilating her body.

He denies killing the journalist, and on Tuesday (local time) told court the hatch slipped and hit her head as they sailed in the strait between Denmark and Sweden last month.

With the vessel at the surface, he said he had crawled out through the hatch and was standing on top, while holding it open to let Wall follow him.

At that moment, he said the submarine was rocked by a wave from another boat.

“I lose my foothold and the hatch shuts,” he told the Copenhagen court, saying Wall was knocked to the floor.

“There was a pool of blood where she had landed.”

A prosecutor also read earlier testimony from behind closed doors in which Madsen said the impact had fractured the journalist’s skull and killed her.

Madsen said he tried to bury her at sea but denied mutilating her body, and added that he had contemplated killing himself while still on board.

The court ordered a psychiatric evaluation and that Madsen be kept in custody for four weeks.

If found guilty, he faces five years to life in prison.

Wall, a 30-year-old freelancer who was researching a story on Madsen, went missing after he took her out to sea in his 17-metre submarine on August 10.

The cause of her death has not been determined.

In court, Madsen denied having amputated her limbs and said he dropped her “whole” body into the water, several hours after her death, after having a sleep because he was “tired and exhausted”.

Peter Madsen was rescued after his submarine sank. Photo: EPA

He admitted that he wanted to “bury her at sea” by attaching metal to the body in order for it to sink.

“I had no contact with the body and didn’t want a dead body in my submarine,” Madsen told the court.

“I put a rope around her feet to drag her out of the hatch,” he said, adding that he was crying during this operation.

He said he considered ending his life but changed his mind because he wanted to see his wife and three cats.

The submarine is one of three Madsen had built and one of the largest privately built ones in the world. It could carry eight people and weighed 40 tonnes fully equipped.

A day after taking Wall out to sea, Madsen was rescued in a navy operation after deliberately sinking the vessel.