Chinese visitor numbers to Japan may decline as much as 20 percent because of a dispute over uninhabited islands in the East China Sea, according to the Japan National Tourism Organization.

“We are very worried,” Mamoru Kobori, the tourism agency’s overseas marketing manager, said by phone yesterday. “There will surely be an impact on the numbers and the only question is how much — 10 percent or 20 percent is possible.”

A slump in demand from its second-biggest overseas market would add to the difficulties facing Japan’s tourism industry as it tries to recovery from last year’s tsunami and nuclear-power crisis. Ctrip.com International Ltd. (CTRP), owner of China’s biggest travel portal, has halted Japan promotions ahead of a weeklong Chinese holiday starting Oct. 1, while travel agents are canceling trips.