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Nearly a decade after it sold its first digital song, iTunes continues to dominate the digital music retail market.

Accoridng to estimates released by the NPD Group Inc. on Tuesday, Apple Inc.'s iTunes has a 63% unit share of the US market for digital music downloads in the fourth quarter.

Amazon's MP3 store comes in a distant second with a 22% market share.

NPD's estimates are based on units, rather than revenue, because iTunes offers fewer discount offers than other retailers. This means that its share of the $2.9billion US download market is likely to be greater than its share of the units sold.

The figures suggest that eight out of ten customers bought their album or song from iTunes in the fourth quarter.

The NPD Annual Music Study 2012 also offered some reassurance to the music retail market, noting that the number of Americans buying digital music downloads has remained stable over the last three years despite the introduction of free streaming sources like Spotify and Pandora.

Downloads and streams recently overtook radio royalties

"There is a belief that consumers don't need to buy music because of streaming options," said NPD's senior vice president of industry analysis Russ Crupnick.

"In fact streamers are much more likely than the average consumer to buy music downloads."

At the end of last month a report from the European Commission Joint Research Centre suggested that illegal downloads didn't harm the number of legal purchases, and in some cases in fact boosted them.