Benesse Holdings Inc., Japan's largest provider of correspondence education for children, said Wednesday the personal information of up to about 20.7 million customers may have leaked from its group company, Benesse Corp.

Based on an in-house investigation, Benesse said it suspects an insider who is not an employee had leaked the information. Police are investigating the matter, it added.

The company, which is known for its Shinkenzemi correspondence courses for schoolchildren, said that to date it has confirmed the leak affects at least 7.6 million customers in its database, including names, genders and birth dates of children.

"We deeply apologize for causing great trouble," Benesse Holdings Chairman and Chief Executive Officer Eiko Harada said at a press conference in Tokyo.

Harada denied he will step down over the incident, but said two directors in charge of the issue will resign once the incident is fully investigated.