South Korea's biggest theft of personal information on credit card holders prompted dozens of top executives at financial firms, including KB Financial Group Inc., to offer their resignations this week as a regulatory probe widened.

Lee Kun Ho, chief executive officer of Korea's largest bank, was among 27 executives who sent resignation letters to KB Financial CEO Lim Young Rok, an official at the Seoul-based company said Monday, asking not to be named in accordance with company policy. An emailed statement from Lotte Card Co. said that nine officials from that company had also offered to quit.

The theft has triggered regulatory and criminal probes this month in a country where credit card use accounts for more than half of total consumer spending. South Korean prosecutors have so far indicted three people on suspicion of stealing names, social security numbers and card data tied to millions of customers of Lotte Card, KB Kookmin Card and Nonghyup Bank.