Standard & Poor's Corp. downgraded $31.5 billion of residential mortgage-backed securities backed by subprime and alternative-A mortgages issued in 2006 and 2007.

The agency has been cutting its ratings on billions of dollars of the securities in recent weeks after raising its expectations for loan losses.

One batch of the downgrades announced Tuesday involved 4% of the first-lien subprime mortgage-backed securities rated by S&P in 2006 and the first half of 2007. The 256 classes from 36 securities deals were initially valued at $24.6 million.

Three-quarters of the classes downgraded had previously experienced ratings cuts. An additional 147 classes from the deals had their ratings affirmed. These securities originally had $21.8 billion of loans behind them.

S&P has its ratings on 2,018 classes from 347 subprime mortgage-backed transactions from the 2005, 2006, and 2007 vintages on warning for further possible downgrade.

The agency cut its ratings on 205 classes of 17 alt-A bond issues with a current outstanding balance of $6.91 billion. Several dozen of the classes had triple-A ratings, and some were slashed to junk territory. S&P says it is reviewing its ratings for 1,948 classes from 246 alt-A transactions.

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