Perhaps as many as four times more sharks than previously thought are being slaughtered to fill the increasing demand for shark fin soup.

Scientists estimate that each year between 26 million and 73 million sharks, weighing up to 2.29 million tonnes, are killed for their fins, which are used in the delicacy that costs $US100 ($150) a bowl in Chinese restaurants.

Figures reported to the United Nations Food and Agriculture Organisation put the number at 0.4 to 0.6 million tonnes a year.

"Our findings confirm that a far larger number of sharks are being caught every year than current databases indicate. The fin trade is continuing to expand and thus the pressure on shark populations is constantly increasing," said fisheries scientist Dr Shelley Clarke of Imperial College, London.

Species most at risk were the blue, hammerhead and silky sharks, say the researchers.

They said official estimates of the number of sharks killed each year may not reflect the true figure because many fish were caught by unregulated fisheries which did not report the catch.

The findings, reported in the journal Ecology Letters, are based on an investigation of shark fins sold at auction in Hong Kong, the world's largest fin market.

The World Conservation Union says 65 out of 373 known shark species are threatened, largely because of the trade in shark fins. Some experts think the number is higher.

Several countries have banned the slaughter of sharks for their fins after public outcries