LG Uplus said that last month's data breach affected a total of 290,000 users, about 110,000 more than initially suspected.
On January 10, the nation's third-largest wireless carrier disclosed that personal data of 180,000 customers, including their names, birth dates and phone numbers, had been breached.
The company said on its website it found personal data of about 110,000 more customers, who had terminated their subscriptions, was also compromised, reports yonhap news agency.
It had been storing the data under the e-commerce consumer protection law.
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The leaked data does not include financial information, it added.
"We are actively cooperating with the investigation by the authorities and the government to determine when and how the personal data was leaked," the company said.
The case is being investigated by the Seoul Metropolitan Police Agency, the Personal Information Protection Commission and the Korea Internet & Security Agency.
"We will strengthen our monitoring system so that such an incident does not happen again and do our best to come up with countermeasures," said the agency.
