U.S. online sales for the 2011 holiday season rose 15 percent to hit an all-time high, according to data firm comScore.

Retail e-commerce sales in November and December reached $37.2 billion, compared to $32.4 billion a year ago, with growth outpacing that of traditional brick-and-mortar stores, comScore reported on Wednesday.

The National Retail Federation expects total retail sales, excluding online business, to be up 3.8 percent for the period.

The jump in online sales was fueled by the growing prevalence of free shipping, which comScore said happened on more than half of the transactions made during the season.

Seven of the 10 business online shopping days during the two-month period took place in December.

(Reporting By Phil Wahba, editing by Dave Zimmerman)

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