KDDI Corp. said Tuesday its "au" mobile phone service had been fully restored by the afternoon, ending 86 hours of service disruptions that affected millions of customers and a range of business activities.

The service outage affected up to 39.15 million mobile connections, disrupting banking systems, the transmission of weather data, parcel deliveries and network-connected cars, among other things, according to Japan's second-largest mobile carrier by subscribers.

But the company's woes are far from over amid mounting criticism of its handling of the unusually long network disruption, including from the regulator, while affected customers called for compensation.

"We deeply apologize for significant inconvenience caused over a prolonged period of time," a KDDI executive said at an online press conference.

Internal Affairs and Communications Minister Yasushi Kaneko speaks at a press conference in Tokyo on July 5, 2022. (Kyodo)

The matter of compensation would be "promptly considered after confirming the extent of the (outage's) impact," the executive said.

According to KDDI's terms and conditions, individual users are eligible for compensation if a service outage lasts 24 hours or longer, although damages such as loss of work resulting from connection issues are unlikely to be covered.

Earlier Tuesday, Internal Affairs and Communications Minister Yasushi Kaneko criticized KDDI for its handling of the disruption, saying the mobile operator failed to provide sufficient information to customers in a timely manner.

The company "has not fulfilled its responsibility as a telecommunications operator," with some customers still unable to make voice calls after KDDI said it had ended restoration work Sunday, Kaneko told reporters.

Taking into account the unprecedented impact of the outage, the government will set up a panel to compile measures to prevent a recurrence, he said.

Referring to the disruption of emergency calls, Kaneko said, "It is extremely regrettable that it has got to the point of potentially threatening people's lives and we are taking the situation seriously."

KDDI first suffered a disruption at around 1:35 a.m. Saturday. The company said Monday afternoon that its services had almost been restored nationwide, but it stopped short of announcing the problems were resolved, saying it still needed time to check its network.

The company said Tuesday its services had been fully restored as of 3:36 p.m., including for its low-cost UQ Mobile brand and lower-priced "povo" mobile customers.

The Japan Meteorological Agency has demanded that KDDI come up with measures to prevent a similar incident after some data distribution from its regional weather observation system was suspended.

The system observes and distributes temperature and precipitation data.

Of the around 1,300 observation stations nationwide, about 550 had stopped transmitting data due to the outage, land minister Tetsuo Saito said, adding the situation had returned to normal.

"Real-time observation data from the system is important for local people and organizations involved in disaster prevention," Saito, the minister of land, infrastructure, transport and tourism, told reporters Tuesday.

Health, Labor and Welfare Minister Shigeyuki Goto said he has requested the country's 47 prefectures to make efforts to prevent any impact on medical services in the event of a similar situation.

Some municipalities had reported that medical facilities were unable to communicate with on-call doctors and drivers who were delivering pulse oximeters to coronavirus patients because of the outage, he said.

Gifu Gov. Hajime Furuta said there had been two cases of people not being able to reach emergency services due to KDDI's outage, but both were resolved without serious consequences.

"To ensure there is no serious interference with medical and nursing-care services, we will work with prefectural governments to secure a stable system," Goto said.

The network failure occurred when a router for voice calls was replaced during regular maintenance, with repair work triggering a concentration of traffic that led the company to reduce user access.

During that time, the carrier experienced a cascade of technical problems that further prolonged the connection difficulties.

The latest service outage follows a system failure at NTT Docomo Inc., Japan's largest mobile carrier, in October last year that lasted around 29 hours and affected at least 12.9 million users.

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KDDI network almost recovered, compensation in focus for disruptions