According to the bank, staff leaked information about share sales involving energy firm Inpex, Mizuho Financial Group and Tokyo Electric Power in 2010.

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Banking shares were among the gainers in Japan, with Sumitomo Mitsui Financial Group up 2.2%, Mizuho Financial Group up 2.9% and Nomura Holdings 2.8% higher.

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The giant Mizuho financial group, for instance, which brings together the house banks of three keiretsu (corporate groups), contains plenty of affiliated life insurers and non-life companies, whose fate is still uncertain.

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Consumer and small-business loans would account for roughly 60% of the merged bank's domestic total, about the same as at Mizuho and Sumitomo Mitsui Financial Group, the other big-four bank.

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