Prices of single-family homes nationwide fell more than twice as fast as expected in September from the prior month, while prices compared to a year earlier rose more slowly than forecast, according a widely watched index of home prices released on Tuesday.

The Standard & Poor’s/Case-Shiller composite index of 20 metropolitan areas declined 0.8 percent in September from August on a seasonally adjusted basis.

Economists polled by Reuters had expected a decline of 0.3 percent.

S&P, which publishes the indexes, also said home prices in the 20 cities index rose 0.6 percent from September 2009, slower than the 1.1 percent expected.