(Bloomberg) -- Nomura Holdings Inc. was excluded as lead manager for several yen bond deals after authorities found market rule violations, the second time this year that Japanese companies cut underwriters over breaches.

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At least four Japanese firms including Toyota Finance Corp., Sumitomo Mitsui Trust Holdings Inc. and Okinawa Development Finance Corp. removed the nation’s biggest brokerage as bond manager after the development, according to underwriter announcements. Japan’s securities watchdog said it had investigated Nomura for suspected market manipulation in government bond futures and found violations.

Nomura’s shares tumbled 8.1% Monday amid a broad selloff in Japanese stocks, with the Topix index dropping 3.5%. Shigeru Ishiba’s victory at the ruling Liberal Democratic Party leadership contest wrong-footed investors who had bet on a boost from more monetary stimulus from his rival.

“We apologize for any inconvenience caused to those involved,” Nomura said in an email sent to Bloomberg. “We will work to prevent a recurrence and restore trust.”

Sumitomo Mitsui Trust’s spokesperson said they made the decision reflecting that the regulators’ penalty could affect the process for the bond issuance. Kensuke Kina, who works for the Okinawa Finance’s fund division, said that there was a possibility that sales would be affected due to the issue involving Nomura’s credibility.

Regulatory troubles have prompted Japanese companies to remove brokerages from debt underwriting mandates in the past. Almost no Japanese company hired Mitsubishi UFJ Morgan Stanley Securities Co. for several months for yen bond deals after regulators punished it over breaching information firewalls earlier this year, Bloomberg-compiled data show.

Nomura’s removal as underwriter comes at a busy time for yen corporate bond deals. Sales of such notes have already reached a record in the fiscal first half started April, exceeding ￥10 trillion ($70 billion). The coming months are typically a busy time for brokerages in the yen primary market.

Hokkaido Electric Power Co. dropped Nomura as a manager for yen transition bonds last week.

--With assistance from Joshua Robinson.

(Adds comments from Nomura and issuers from the fourth paragraph)