The Internet research firm said fourth-quarter Web sales, excluding travel purchases, will rise to $13.8 billion from $10.8 billion last year. The forecast is based on growth rates recorded so far this year, ComScore said.

Internet sales have been increasing by at least 20 percent each year as more people gain access to the Internet and become comfortable with online shopping. Economic sluggishness is not affecting the growth of online sales as much as traditional store sales because more people are using the Web for shopping, said Ken Cassar, a Jupiter Research analyst.

"The online sales are growing so quickly that the negative impact is not easy to see," Cassar said.

ComScore expects consumers to begin their Internet holiday shopping on the Monday after the Thanksgiving weekend, when traditional stores kick off the season. That's when people head back to work and use faster Web connections in their offices to make purchases, said ComScore spokesman Max Kalehoff.