Moody's Investors Service Inc. said Tuesday it has changed the outlook for the Aa1 senior long-term debt ratings on NTT DoCoMo Inc., Japan's dominant mobile phone operator, to negative from stable.

"The change reflects Moody's concern over the future of DoCoMo's competitive advantage in third-generation (3G) telecommunications services, which are important for maintaining the long-term stability of the company's cash flow generation," the U.S. credit-rating agency said.

In 2001, NTT DoCoMo became the world's first operator to introduce a W-CDMA network, or a 3G technology based on IMT-2000 standards.

But growth in subscribers to the service, known as FOMA for Freedom Of Mobile multimedia Access, has been below expectations and numbered only 149,000 as of the end of November, Moody's said.

The problem is due largely to its relatively weak attractiveness, such as original geographic coverage, the battery life of handsets and service content, compared with the company's second-generation technology, it said.

While NTT DoCoMo's 2G services are popular and meet the needs of its customers, competitors have introduced their own 2.5G and 3G services, it said.

Consequently, NTT DoCoMo may face greater challenges in securing for FOMA the solid market position it has acquired for its 2G services, Moody's said.

"This, in turn, means the company's ability to generate stable and exceptionally rich cash flow, which supports its Aa1 rating, may also be increasingly challenged over the intermediate term," it said.