Lotte Card Co.'s data breach was found to have been more severe than initially thought, financial authorities said Wednesday, noting personal information, such as card records, of a large portion of customers may have been leaked.

Earlier this month, Lotte Card, with some 9.6 million members, disclosed a cyber breach. Since then, the financial watchdog and the card company have been struggling to gauge the damage caused by the hacking.

According to the officials at the Financial Supervisory Service, the size of the data leak and the damage were found to have been more serious than earlier thought.

"We are working to complete the probe (into the data leak) and may announce the result this week," an FSS official said.

Lotte Card's data breach is the latest in a series of massive cyber breach cases in the financial sector. Earlier, SGI Seoul Guarantee had suffered a similar hacking.

Lotte Card's cybersecurity lapse also came as FSS Gov. Lee Chan-jin has been calling on financial firms to beef up their cybersecurity and better safeguard customers. (Yonhap)