NEW YORK (Reuters) - Prices of existing single-family homes slumped for the 18th month in a row in January, for a record annual drop, according to Standard & Poor’s/Case-Shiller home price index released on Tuesday.

A "for sale" sign advertises a reduced price for a home for sale in Portland, Oregon December 26, 2007. REUTERS/Richard Clement

The composite month-over-month index of 20 metropolitan areas fell 2.4 percent to 180.65 from December, bringing the measure down 10.7 percent from a year earlier and 12.5 percent from its July 2006 peak.

The 12-month drop was the latest in a string of records for the statistic set as the housing market slumped. The previous annual drop, in December, was 9.1 percent.

The latest annual decline was just over half of the 20 percent total decline expected by S&P’s chief economist, David Wyss, said David Blitzer, a managing director at S&P.

Wyss expects prices to hit bottom in early 2009, Blitzer said in a conference call.

“Unfortunately, house prices continue to decline and the decline continues to be really nationwide,” said Blitzer, who chairs S&P’s index committee.

Falling home prices have broken the back of the U.S. housing market, which during the housing boom was increasingly financed with risky loans. Homeowners, especially those with little or no equity in their property, are facing a record foreclosures, threatening the economy that some economists say is in recession.

House price depreciation accelerated in February, according to FBR Investment Management analyst Michael Youngblood, citing data from data provider LoanPerformance.

S&P said its composite month-over-month index of 10 metropolitan areas fell 2.3 percent to 196.06 for an 11.4 percent year-over-year drop. That was also a record, surpassing the previous month’s drop of 9.8 percent.

Home prices in Las Vegas and Miami fell the most of any region, at 19.3 percent year-over-year. Phoenix, San Diego and Los Angeles also suffered double-digit drops, the S&P index shows.

“The weakness is not contained to the bubble areas,” said Michelle Meyer, an economist at investment bank Lehman Brothers in New York. “It has spread to the rest of the nation.”

Another housing index published on Tuesday, from the Office of Federal Housing Enterprise Oversight, showed home prices fell nationally about 1.1 percent from December to January, and 3 percent compared with a year ago.

The OFHEO index tracks data from Fannie Mae and Freddie Mac, the two largest providers of funding for U.S. home loans. The S&P indexes include home sales financed by loans that may fall outside Fannie Mae and Freddie Mac guidelines, but are limited to 20 metropolitan areas.