Even including off-balance-sheet assets, the three banks still have Basel leverage ratios above 3 percent, reckons Nana Otsuki, a banking analyst at Merrill Lynch Japan Securities.
But this Basel ratio is only meant to be a minimum; the United States, Britain and others are pushing for stricter capital rules. Ratios as high as 4-6 percent have been mentioned, although the additional buffers could be defined less stringently that the first 3 percent.
And even if the global minimum remains the same, countries can set their own higher thresholds, so global investors might press all major banks to meet the same standards as, say, U.S. and UK lenders. This could mean the megabanks would have to retain more earnings, sell shares to raise capital or both.
"We want a buffer of 0.5-1 percentage point" above requirements, said a senior executive at one of the top banks.
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A 1-point increase in the leverage ratio would mean each megabank would need to set aside 1.5-2.5 trillion yen ($15-24 billion) more capital.
For now, the potential capital rules will not be a big impediment to banks' shareholder returns, said banking analyst Toyoki Sameshima at BNP Paribas Securities. The banks this year will likely increase dividends or buy back shares, and they have various ways of raising capital and shedding assets, he said.
MUFG historically has preferred to pay out dividends, Tanaka said, but he did not rule out the possibility of a share buyback as well.
An SMFG spokesman said that in considering dividend payments, "we need to make proper shareholder returns in consideration of our capital level, dividend ratio and other things against the background of Basel III."
The megabanks have some room to share the wealth with investors.
Their dividends and buybacks amount to 20-25 percent of net income, one analyst said. They have the capacity to raise their redemption ratios to 30 percent, the average for Japan's regional banks, and they might do so, said the analyst who asked not to be identified due to company policy.
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But experience has taught the megabanks to be careful. They showered money on shareholders, including 150 billion yen each by MUFG and Mizuho, in 2007 - only to have the financial crisis hit, forcing them to tighten up again.
"They need to be cautious about capital outflows until the direction of the leverage ratio debate becomes clear," said a senior official at Japan's Financial Services Agency.