YICHANG, April 20 (Xinhua) -- The China Three Gorges Project Corporation announced on Friday that the dual-track ship lock of the Three Gorges Dam would fully reopen to traffic on May 1, one month ahead of schedule.

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 raising of the lock beds was necessary because the water level behind the dam is set to rise to 175 meters.

The operation began on the southern route, which reopened on Jan. 20, and work on the northern track started the same day.

The shipping capacity is down by 60 percent.

Work on the northern track was almost completed and workers have been applying rust prevention treatment to the giant door of the ship lock.

The Three Gorges Project, the world's largest water control facility, is located on the middle reaches of the Yangtze River, China's longest and one of the country's most important inland waterways. It boasts a 185-meter-high dam, which was completed on June 20, 2006.

The lock, 6.4 km long and costing 6.2 billion yuan (775 million U.S. dollars), was built into mountainous terrain on the northern bank of the Yangtze and has been the only navigable route past the dam since 2003.

Construction of the lock began in April 1993. Trial operations began ten years later and it became fully operational in July 2004. By the end of 2005, about 190,000 vessels carrying 89 million tons of cargo and more than 1.88 million passengers had passed through it.

In accordance with the original schedule, the entire raising ofthe lock beds will be finished on June 30, and two-way traffic will resume the same day.