Prices of existing single-family homes dropped further in August in their worst erosion since June 1991, according to the Standard & Poor's/Case Shiller national home price index.

S&P said its composite month-over-month index of 10 major metropolitan areas declined 0.8 percent in August to 214.35, for a 5 percent year-over-year drop, S&P said in a statement on Tuesday. The worst decline in the index was a 6.3 percent drop in April 1991.

The composite month-over-month index of 20 metropolitan areas fell 0.7 percent to 197.16 in August from July, bringing the measure down 4.4 percent from the year-ago period,

Both indexes have registered falling prices since the start of this year as soaring inventories of homes and tight lending conditions worsened the housing slump.

The credit crisis deepened in August when defaults on risky loans drove investors away from securities which fund lending to even the best-rated borrowers.