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.

He said the new plan will cut the company's average revenue per user, but declined to specify how by much.

The revenue per user of KDDI's au service declined to 6,380 yen in the fourth quarter from 6,600 yen in the third quarter, while the figure at NTT DoCoMo Inc. dropped to 6,530 yen from 6,670 yen and at Softbank to 5,210 yen from 5,560 yen.

Takahashi also said KDDI will announce a new service by the end of this year, but would not give any details.

In January, Softbank aggressively cut its basic monthly rates — the lowest was reduced to 980 yen — to get customers from its competitors in the saturated mobile-phone market. It had the most new subscribers in June for the second month in a row.

Softbank, headed by billionaire Masayoshi Son, added 204,800 subscribers last month, bringing the total number of customers to 16.4 million. KDDI got 133,200 new users in June to bring its total to 28.7 million subscribers and NTT DoCoMo added 88,800, for a total of 52.8 million.

NTT DoCoMo, Japan's biggest mobile phone carrier, countered Softbank's price cuts with an announcement in June of its own price reductions. The carrier will slash its monthly rates by up to 50 percent starting in August.

NTT DoCoMo President Masao Nakamura said at a news conference in Tokyo to make the announcement that the discount package would cut 20 yen billon from sales in the firm's business year to March, but could attract up to 5 million new customers. NTT included the predicted loss in its annual forecast.

The focus now is how Softbank will react. Son has promised his company will outbid its rivals for the cheapest subscription rates.

Softbank spokesman Yuki Akazawa would only say new cuts are being considered.