New York ? U.S. home prices fell in October for the 10th consecutive month, posting their largest drop since early 1991, according to a key index released Wednesday.

The record 6.7 percent slide in the Standard & Poor’s/Case-Shiller home price index also marked the 23rd consecutive month that prices either fell or grew more slowly than the month prior.

“No matter how you look at these data, it is obvious that the current state of the single-family housing market remains grim,” said Robert Shiller, who helped create the index, in a statement.

The previous record decline was 6.3 percent, recorded in April 1991. The index tracks prices of existing single-family homes in 10 metropolitan areas.

It is considered a strong measure of home prices because it examines price changes of the same property over time, instead of calculating a median price of homes sold during the month.

Home prices could fall another 10 percent during the next 12 to 18 months before bottoming out, said Patrick Newport, an economist with financial consultancy Global Insight, in an interview.

Newport said four of the largest groups currently trying to sell homes – banks holding foreclosed properties, homebuilders, speculators and unemployed consumers – typically are flexible about lowering house prices because they need to get rid of the property.

Sales of homes likely will start to rebound late in 2008, with price appreciation to follow, Newport said.

A second, broader Case-Shiller index, which measures 20 metropolitan areas, fell 6.1 percent in October. Among the 20 areas used in the broader index, 11 posted record year-over-year declines and all 20 declined in October compared with September.

Leading the index lower was Miami, where prices fell 12.4 percent in October compared with the same month last year. That led it to surpass Tampa, Fla. as the worst-performing city. Tampa posted a year-over-year loss of 11.8 percent.

Besides those two cities, Detroit, Las Vegas, Phoenix and San Diego also posted double-digit year-over-year declines.