Japan's financial regulator will widen a probe next week into whether all three of the country's big banks, including top lender Mitsubishi UFJ Financial Group, conducted transactions with Japanese gangsters, known as the yakuza.

Second-ranked Mizuho Financial Group has already come under regulatory scrutiny and has sanctioned 54 executives, including its president, following revelations that a consumer finance affiliate extended more than $2 million in loans to people tied to organized crime.

Mitsubishi UFJ, Mizuho and Sumitomo Mitsui Financial Group Inc were given notice on Tuesday of the inspections, the head of the Financial Services Agency's investigation unit told a parliamentary committee.