A day after saying big U.S. banks probably needed to raise only one-fourth the capital demanded by the government, Standard & Poor's said the nation's banking crisis has merely entered a new phase and might not end before 2013.

The credit rating agency said the industry is being propped up by hundreds of billions of dollars of government support, especially for lenders considered too important to the financial system to fail.

While efforts to spur lending, take bad assets off banks' balance sheets, and restart the market for packaging and selling securities may help the sector, S&P said banks will have a tough time surviving absent a bigger capital cushion than regulators require.

There's nothing to say that this banking crisis can't go on for another three or four years, S&P Managing Director Tanya Azarchs said.

S&P did not immediately return a request for comment.

On Tuesday, S&P said major U.S. banks need to raise about $18 billion of capital to protect themselves from the economic downturn, though this amount could grow if conditions worsen.

The amount is well below the $74.6 billion that the government last week ordered 10 of the largest U.S. banks, led by Bank of America Corp and Wells Fargo & Co , to plug potential capital shortfalls.

These 10 banks were among 19 subjected to government stress tests to gauge their readiness to withstand a particularly severe recession in 2009 and 2010.

The other nine, including JPMorgan Chase & Co and Goldman Sachs Group Inc , got clean bills of health when stress test results were released on May 7.

S&P on May 4 said it may lower its ratings for 23 U.S. banks and thrifts, including 10 that underwent stress tests, citing concern about the industry's capitalization.

It said the 23 companies had at least a 50 percent chance of being downgraded within 90 days.

(Reporting by Jonathan Stempel; Editing by Richard Chang)

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