KDDI Corp. said Thursday it plans to cut its basic monthly mobile-phone fees by 50 percent starting in September if users sign a two-year-contract — a decision that could trigger an industry price war.

"We have been performing well since the introduction of number portability, but that effect is lessening so we want to invigorate the market once again," Makoto Takahashi, an associate senior vice president, told a news conference in Tokyo. Number portability allows subscribers to change their service provider without changing their phone number.

Takahashi said the discount plan would take a 20 billion yen bite from sales, as was reflected in this year's earnings forecast. He would not say how many new subscribers the company hopes to get with its new plan.