Over-exploitation of groundwater is believed to be the cause of the sinking, according to an investigation by land authorities from Shanghai and Zhejiang and Jiangsu provinces, Xinhua news agency said.

"Extended pumping of groundwater has already led to subsidence and cracks in an area of about 100,000 square kilometers (40,000 square miles) south of the river," said Guo Kunyi, a researcher in charge of the investigation.

"And a nearly 10,000-square-kilometer area in the delta has sunk for more than 20 centimeters (eight inches)."

Guo said Shanghai, the place where the subsidence was first spotted, suffers the most from it.

The findings of the study were contained in a report released by the Ministry of Land and Resources. Xinhua gave no breakdown on the costs.

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