Standard & Poor's downgraded 187 classes of pre-crisis jumbo residential mortgage-backed securities previously rated AAA.

The credit ratings agency downgraded 259 classes total on 39 jumbo RMBS deals issued between 2003 and 2007.

"The downgrades were primarily driven by increased losses due to an increase in our default and loss multiples at higher rating levels," S&P said. It also found higher redefaults on previously cured loans and longer timelines required to liquidate nonperforming mortgages.

Of the 259 downgrades announced Tuesday, 34 eventually went to junk status from investment grade.

In 10 deals issued by Merrill Lynch between 2003 and 2005, S&P downgraded 35 senior classes of AAA-rated shelves and seven subordinate classes given the highest rating.

For nine Sequoia deals issued by Redwood Trust (RWT) over the same period, S&P downgraded 40 AAA classes – one of them all the way to junk status BB+.

S&P lowered ratings on 30 AAA classes of Wells Fargo (WFC) securities issued in 2004 and 2005. The highest ratings on 2006 Wells Fargo jumbo mortgage bonds remained the same because the agency said it expects these loans to pay off within in the next year.

Another 75 AAA classes were downgraded from 10 deals issued by several banks including Citigroup (C), GMAC, JPMorgan Chase (JPM) and others.