A customer advocacy group said Monday it will ask the financial watchdog to probe banks and credit firms over a recent massive leak of customers' personal information.

The Financial Consumer Agency said it will specifically request the Financial Supervisory Service (FSS) next month to investigate six financial firms -- Standard Chartered Bank Korea, Citi Bank Korea, Kookmin Bank, NongHyup Bank, KB Kookmin Card and Lotte Card

-- on behalf of customers whose private data were leaked.

Industry sources said Sunday that personal information, including bank account numbers, addresses and credit ratings, of some 20 million clients had been stolen. Part of the leak occurred at local banks that shared their customer data with their affiliated credit card firms, such as KB Kookmin, Nonghyup and Lotte. Customers and authorities are most concerned that the leaked information may have gone to financial scammers.

In December, personal data of some 130,000 customers of Standard Chartered Bank Korea and Citi Bank Korea was stolen, the largest number in the history of the banking sector in South Korea.

The FSS had already been under criticism for its lax supervision of local financial firms. Hundreds of investors are reeling from the losses in their investments in short-term financial instruments issued by Tong Yang Group, who sold the investment products knowing that the company was deeply troubled financially.

The state auditor started auditing the FSS and the Financial Services Commission (FSC), the top regulator, on Monday in a bid to hold them accountable for their lax supervision of Tong Yang Group. (Yonhap News)