Standard & Poor's reiterated on Thursday it sees a real risk that future U.S. government deficits may meaningfully miss discussed targets and that there is a 50-50 chance the U.S. AAA credit rating could be cut within three months, perhaps as soon as August.

The deficit reduction debate is coming up against an August 2 deadline when the $14.3 trillion limit on America's borrowing capacity is exhausted, putting in jeopardy payments on U.S. Treasury debt as well as paychecks for federal employees and soldiers.

If an agreement is reached to raise the debt ceiling but nothing meaningful is done in terms of deficit reduction, the U.S. would likely have its rating cut to the AA category, S&P said.

While banks and broker-dealers wouldn't likely suffer any immediate ratings downgrades, we would downgrade the debt of Fannie Mae, Freddie Mac, the 'AAA' rated Federal Home Loan Banks, and the 'AAA' rated Federal Farm Credit System Banks to correspond with the U.S. sovereign rating, S&P said in its report.

We would also lower the ratings on 'AAA' rated U.S. insurance groups, as per our criteria that correlates insurers' and sovereigns' ratings, the firm said.

However, S&P said it sees a failure to reach an agreement on raising the debt ceiling and reducing deficits as the least likely scenario, adding that in such a case the global financial markets would be in turmoil and likely shove the U.S. economy back into recession.

In such a hypothetical case, it envisages the U.S. Treasury curtailing spending sharply and the U.S. Federal Reserve launching another round of quantitative easing to help prop up the economy.

Under this scenario, we expect that interest rates could rise--say, 50 bps on short-term rates and double that on the long end--though this may depend on whether Treasuries would lose their status as the safe haven that investors have historically perceived them to be, or whether physical assets such as gold would benefit from such a flight to quality, S&P said.

It added that either way, corporate borrowers would likely see yield spreads widen while equity markets and the U.S. dollar would likely suffer.

The outline of potential knock-on effects of a U.S. credit rating downgrade were first reported by Market News International.

As Aug 2 approaches, the U.S. Treasury market has grown sensitive to news on the potential for the U.S. to actually default or, even if Washington can reach a deal to avoid default, a downgrade based on longer-term fiscal conditions.

The S&P's latest comments led to selling in longer-dated Treasuries, with the 30-year bond briefly falling a full point in price.

(Reporting by Emily Flitter and Daniel Bases; Editing by Theodore d'Afflisio)

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