Standard & Poor's cut its ratings on 1,862 classes of RMBS backed by prime jumbo mortgages issued from 2002 through 2004.
The rating agency said projected losses because of increased delinquencies are expected to be greater than the amount of collateral.
Jumbo loans, issued to homeowners needing at least $417,000 in financing, have seen a sharp rise in delinquencies. However, the total delinquency rate remains sharply lower than those of other types of loans.
The downgraded classes are from 416 RMBS deals. S&P also affirmed the ratings on 4,390 classes from 414 transactions.