Nobody was injured and sea traffic was directed elsewhere.

But a Marine Department investigation has been launched into what caused the accident, and why the vessel's master abandoned ship without turning off the engine.

The officer said the master had been asked to go back on board to turn off the engine.

"But he was scared to go back because the vessel was listing badly and taking in water. He was worried he might be trapped inside if it capsized," the officer said.

The officer said it was fortunate the ship was in a marine fairway that was not busy and on which ships ran slowly. It was carrying 2,800 tonnes of cement from Nansha port in Guangdong province to Indonesia, and had refuelled in Hong Kong.

It was about one kilometre east of Cheung Chau when it started listing shortly after 10am.

A Marine Department spokeswoman said the ship had "suddenly begun to list", while a police source said: "The cargo was probably moving on board, which caused the vessel to lean significantly to one side."

The spokeswoman said the boat's captain issued a distress signal and the crew members gathered on one side of the ship to await rescue. They were brought onto a police launch.

A Marine Department officer said the vessel was, at one point, listing at a 45 degree angle. The boat powered on for about an hour before its engine cut out. It then drifted a further kilometre as attempts to bring it under control using two tugboats failed.

The boat eventually grounded at Tung Wan Tsai about 1.30pm.

Problems continued last night as oil spilled out of the vessel, forming a slick about 100 metres in length and three metres wide.

Floating booms were deployed by the department's pollution control unit in an effort to stop the oil from spreading.

A spokesman said that the owner of the Vietnamese-flagged vessel had been given 10 days to remove it.