Standard & Poor's said that it will change its method of rating securities backed by U.S. residential mortgages granted before 2009.

The adjustment will result in more downgrades than upgrades, New York-based S&P said today in a statement. In a sample of securities studied, two-thirds of the rating increases would involve raising debt to CCC grades from CC, both among its lowest rankings, the firm said. Certain types of AAA rated bonds will all be lowered to AA+ or less, according to the statement.

Much of the $1 trillion of so-called non-agency home-loan bonds without government backing have been cut to speculative grades of less than BBB- after record defaults on the underlying mortgages and declines in home prices. S&P’s latest review will be completed over the next six months, it said.

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