Banks have started trial gold imports directly into the Shanghai free trade zone ahead of the launch of a gold exchange there, threatening to further obscure the level of buying by the world's top consumer.

The bulk of gold bought by China used to flow through Hong Kong, making its export data a useful proxy for Chinese demand as Beijing treats data about imports of the precious metal as a state secret.

But Reuters calculations using export numbers from data provider Global Trade Information Services show that China's direct gold imports jumped to nearly 150 tonnes in the first quarter.