From November 1 through November 18, $10.1 billion was spent online, a 16 percent increase versus the corresponding days last year, comScore reported. So far, November 8 has been the heaviest online spending day of the season so far at $829 million, it added.

"Recent 5-year highs in consumer confidence and early retailer promotions appear to be serving as wind in the sails for the beginning portion of the holiday season, with consumers opening up their wallets early and often," comScore Chairman Gian Fulgoni said in a statement.

E-commerce accounts for 5 percent to 10 percent of total consumer spending in the United States. However, it is growing much faster than bricks-and-mortar retail as shoppers are lured by low prices, convenience, faster shipping and wide selection.

ComScore expects online retail spending to rise 17 percent to $43.4 billion this holiday season. That is above the 15 percent increase last season and ahead of the retail industry's expectation for a 4.1 percent increase in overall spending this holiday.

Black Friday, the day following Thanksgiving, has traditionally been the busiest shopping day in the United States. However, some big retailers have been trying to grab market share by running holiday promotions earlier.

This year, Amazon.com Inc, the world's largest Internet retailer, launched its Black Friday deals on November 19, five days before Black Friday.

ChannelAdvisor, which helps merchants sell via eBay, Amazon.com and other websites, reported that client same-store sales on Amazon jumped 41.5 percent during the first 19 days of November, versus the same period a year earlier.

Client sales on eBay rose 24.5 percent in the same period, ChannelAdvisor also said.

"The eBay and Amazon marketplaces are looking the strongest heading into the holiday," Scot Wingo, chief executive of ChannelAdvisor, wrote in a blog on Wednesday.

(Reporting By Alistair Barr; Editing by Gunna Dickson, Bernard Orr)

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