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The five-level dual-track ship locks of the Three Gorges Dam, the largest of its kind in the world, passed engineering tests after 13 years of construction, experts announced yesterday.

"The permanent ship lock demands the most sophisticated technology in the entire Three Gorges project," said Zhang Guangdou, an academician on both the Chinese Academy of Sciences and the Academy of Engineering.

"Tests have shown the lock would ensure safe navigation for ships when the water level behind the dam rises from 156 to 175 meters. This means the two-way traffic can resume on May 1, two months ahead of schedule," he said.

"The engineering, metal structures, machinery and electronic equipment of the lifting facility for the 113-meter drop between the upstream and downstream have reached international standards," said Niu Xinqiang, vice director of the Yangtze River Water Resources Committee.

The lock, 6.4 km long and costing 6.2 billion yuan (US$775 million), started to be built into mountainous terrain on the northern bank of the Yangtze in 1994 and has been the only navigable route past the dam since 2003.

The four-level operation began in 2004. The lock has been restricted to one-way traffic, alternating every 24 hours, since September last year when work began to raise the beds of the two uppermost tiers of the lock from 131 to 139 meters.

The operation began on the southern route, which reopened on January 20, and work on the northern track started the same day. The Three Gorges Project is located on the middle reaches of the Yangtze River, China's longest and one of the country's most important inland waterways.