U.S. online retail spending reached a record-breaking $43.4 billion in the fourth quarter, fueled by strong holiday spending, Comscore reports.

Spending in the fourth quarter was up 11 percent over a year ago, representing the fifth consecutive quarter of positive annual growth and the second quarter of double-digit growth rates in the past year.

In 2010, holiday spending online was bolstered by the first-ever billion-dollar day on record, and others surpassing $900 million. The strong growth rates are a huge swing from two years ago, when fourth-quarter sales fell by 3 percent.

The bulk of online sales occur among the top 25 online retailers, which accounted for 68.4 percent of dollars spent.

As the largest e-commerce provider, Amazon turned in a fourth-quarter performance that mirrored Comscore's findings. Amazon reported its first $10 billion quarter ever and saw year-over-year growth rates of 40 percent (its best growth rate since 2000).

In a release, Comscore Chairman Gian Fulgoni predicted that the increased spending levels will continue this year: "We anticipate that the progress we've seen in the past year as we climbed out of the recession will continue with sustained double-digit growth rates in 2011."

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