A major system outage that affected Japan's largest mobile carrier NTT Docomo Inc. last week constitutes a "serious incident" under the telecommunications law and the communications ministry is considering issuing an administrative order, ministry officials said Tuesday.

NTT Docomo will be required to report to the ministry by Nov. 13 the cause of the failure, which left some 2 million customers without voice and data services at one point, and measures to prevent such incidents from occurring in the future, the officials said.

The law on telecommunications business stipulates that for a call service that handles emergency calls, a glitch that affects more than 30,000 people for more than an hour is considered a serious incident.

The communication failure occurred around 5 p.m. Thursday during work on NTT's network. Some users completely lost access to call and internet services for more than two hours before the system was fully restored at 10 p.m. Friday.

NTT Docomo said it "will respond sincerely" if the ministry notifies it that the case is considered a serious incident.

It said earlier its system responded in an unexpected way during the work on Thursday, and that data that backed up during the outage overwhelmed the system when the company tried to restore services.

In January 2019, the Ministry of Internal Affairs and Communications instructed SoftBank Corp., Japan's third-largest carrier by user base, to improve its response after faulty software provided by Telefonaktiebolaget LM Ericsson caused an outage that affected more than 30 million subscriber accounts for more than four hours.

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