Sumitomo Mitsui Financial Group and its securities unit were penalized after Japan’s financial regulator found that the unit had engaged in stock market manipulation.

The Financial Services Agency told the nation’s second largest banking group to improve its oversight of SMBC Nikko Securities Inc., noting the seriousness of market manipulation allegedly committed by the brokerage, according to a statement Friday. The firms have to submit remedial measures to the regulator by Nov. 7.

The financial watchdog also ordered SMBC Nikko to suspend operations related to block offers for three months, in a fresh blow to the company’s reputation. On top of this, it told the brokerage to put together a plan to prevent the recurrence of breaches over client information sharing rules, as well as demanded clarification of management responsibilities.