Single-family home prices dropped in March, dipping below their 2009 low, as the housing market remained bogged down by inventory and weak demand, a closely watched survey said on Tuesday.

The S&P/Case Shiller composite index of 20 metropolitan areas declined 0.2 percent in March from February on a seasonally adjusted basis, in line with economists' expectations.

The price index was below the low seen in April 2009 during the financial crisis. The glut of houses for sale, foreclosures, tight credit and weak demand have kept the housing market on the ropes even as other areas of the economy start to recover.

The 20-city composite index was at 138.16, falling below the 2009 low of 139.26.

This month's report is marked by the confirmation of a double-dip in home prices across much of the nation, David Blitzer, chairman of the index committee at S&P Indices, said in a statement. Home prices continue on their downward spiral with no relief in sight.

Eight cities fell 1 percent or more in March, while Washington was the only city where prices increased on both a monthly and yearly basis. Prices in the 20 cities fell 3.6 percent year over year, topping expectations for a decline of 3.3 percent.

The declines sustained in the last 12 months have almost erased the gains of the previous 12 months. The housing market is treading backward, but not drowning, said Cary Leahey, economist and managing director at Decision Economics in New York.

In the first quarter, the national index fell 1.9 percent on a seasonally adjusted basis, compared to a decline of 1.8 percent in the previous quarter. On a non-adjusted basis, they fell by 4.2 percent in the quarter. Nationally, home prices are back to their mid-2002 levels, the report said.

Financial markets saw little reaction to the data, with U.S. stock index futures pointing to a higher open on optimism that new aid for Greece from the European Union was on the horizon.

(Reporting by Leah Schnurr, additional reporting by Ellen Freilich, editing by W Simon )