Japan's Financial Services Agency (FSA) will inspect three major banks, including giant Mitsubishi, over possible transactions involving yakuza (above) organized crime gangs.
One of them, Mizuho Financial, has already admitted senior managers knew three years ago it had lent 200m yen ($2m; £1.28m) to Japanese criminal gangs, but did not take action.
The bank had previously denied knowing about the loans to yakuza gangsters.
Yasuhiro Sato, the group's president, said an internal investigation showed former president Satoru Nishibori was made aware of the loans when they first came to light in 2010.
Meanwhile, Japan's Financial Services Agency has asked the group to submit a business improvement plan, which includes a "workable and concrete policy to prevent any recurrence" of such incidents in the future.
It has also asked it to implement a compliance structure and enhance its internal auditing functions.
Sumitomo Mitsui is the third involved.
The yakuza gangs are not actually illegal. But like the Italian mafia or the Chinese triads, they are involved in unsavory activities such as gambling, drugs and prostitution, as well as operating protection rackets.
The origin of the yakuza is a matter of some debate. Some feel that its members are descendants of the 17th-century kabuki-mono (crazy ones), outlandish samurai who reveled in outlandish clothing and hair styles, spoke in elaborate slang, and carried unusually long swords in their belts.
The kabuki-mono were also known as hatamoto-yakko (servants of the shogun). During the Tokugawa era, an extended period of peace in Japan, the services of these samurai were no longer needed, and so they became leaderless ronin (wave men).
Without the guidance of a strong hand, they eventually shifted their focus from community service to theft and mayhem.
Modern yakuza members refute this theory and instead proclaim themselves to be the descendants of the machi-yokko (servants of the town) who protected their villages from the wayward hatamoto-yakko.
The official yakuza history portrays the group's ancestors as underdog folk heroes who stood up for the poor and the defenseless, just as Robin Hood helped the peasants of medieval England.
Lawyers hired by Mizuho to look into the transactions said "many officials and board members were aware of, or were in a position to be aware of, the issue".
But the lawyers' report also said that Mizuho failed to recognize it as a problem, believing that the compliance division "was taking care of it".
The company said 54 former and current executives would be punished, including Mizuho bank chairman, Takashi Tsukamoto, who is resigning his position but will remain as head of the parent company.