Japan Post Co. announced Wednesday that it has lost documents containing the personal information of 214,000 clients who had traded national government bonds or opened trust funds at post offices nationwide.

The documents included the clients' names, Japan Post Bank Co. account numbers and the amount of their trade.

The company has an in-house rule that such records must be kept for 10 years after the transactions, but it is believed that the documents were discarded by mistake before the period expired.

It is highly unlikely that the documents were leaked to a third party, Japan Post said.

According to the firm, documents were found to be missing at 6,389 post offices, about 30% of a total of 19,816 post offices nationwide that handle government bonds and trust funds.

"We not only caused worries among our customers but also damaged the trust towards post offices," said Atsushi Hasegawa, executive officer of Japan Post, during a news conference. "We apologize and we will make utmost efforts to prevent recurrence."

In November last year, the firm said four post offices in Gunma, Saitama, Chiba and Yamaguchi prefectures had lost records containing personal information on a total of 318 clients. Japan Post Bank, which commissions Japan Post for teller work, launched a nationwide investigation to see whether there were other similar cases.