WASHINGTON, Sept 30, 2008 (AFP) - Home prices in 20 major US cities plunged a record 16.3 percent in July from a year ago, according to a Standard & Poor's/Case-Shiller survey released Tuesday.

The index for the 20 metropolitan areas fell 0.9 percent for the month of July, in the 19th consecutive monthly drop.

Since peaking in July 2006, months after the housing bubble began to collapse, the prices have fallen by an unadjusted 19.5 percent in the big metropolitan areas.

The index for home prices in the 10 largest cities also posted a record 12-month decline of 17.5 percent in July. On a monthly basis, prices fell 1.1 percent.

In June, home prices in the 20 largest metropolitan areas fell 15.9 percent from a year ago, while those in the 10 largest cities dropped 17.0 percent; both declines had been records.

The monthly S&P/Case-Shiller Home Price Indices provide a snapshot on urban housing price trends. They are calculated using a three-month moving average and published with a two-month lag.