The Japan Post group likely violated the law or internal rules while selling about 10,000 insurance contracts, according to sources.
The number is far bigger than the 6,327 contracts highlighted in an interim report in September on inappropriate sales practices at the group.
The final report on the wrongdoing is scheduled to be released Wednesday by Japan Post Holdings Co., along with subsidiary Japan Post Insurance Co. and mail delivery unit Japan Post Co.
The Financial Services Agency and the internal affairs ministry are expected to issue an administrative penalty within a month of examining the report.
The focus is on whether group leaders, such as Japan Post Holdings President Masatsugu Nagato, will resign.
The group plans to hold a news conference Wednesday to be attended by Nagato, Mitsuhiko Uehira, president of Japan Post Insurance, and Kunio Yokoyama, president of the mail delivery unit.
A special investigation committee led by lawyer Tetsuo Ito will also release a report on the causes of the sales irregularities and preventive measures.
The group said back in July that sales irregularities may have affected 183,000 contracts concluded with 156,000 policyholders.
Of the policyholders, about 120,000 have been interviewed so far, the sources said Sunday, adding the results indicate that violations of laws or internal rules are likely to have affected about 10,000 contracts.
Probes into group employees related to the inappropriate practices are also underway.
The group expects that eventually the irregularities will be confirmed to apply to only a portion of the 10,000 contracts, the sources said.
The September report found likely problems with the 6,327 contracts based on the results of interviews with some 60,000 policyholders.
In its upcoming report, the special committee is expected to say that inappropriate sales practices spread because of excessively ambitious sales targets and a failure to share information on customer complaints among group entities.
The committee is expected to also attribute blame to the board for insufficient corporate governance, and request drastic measures to prevent similar wrongdoing in the future.