By Nandita Bose

NEW YORK, Dec 1 (Reuters) - U.S. online sales grew a smaller than expected 8 percent on Cyber Monday after web promotions before and during the Thanksgiving weekend robbed business from what has traditionally been the busiest day of the year for Internet shopping.

As of 6 p.m. EST, data from IBM Digital Analytics Benchmark showed Cyber Monday sales grew sharply slower than comScore data over the weekend which showed a 32 percent rise in online orders on Thanksgiving and 26 percent on Black Friday.

Cyber Monday sales were projected to rise between 13-15 percent.

The results highlight the waning importance of traditional shopping days like Cyber Monday and Black Friday, which until a few years ago kicked off the holiday shopping season.

To compete with each other and Amazon, most U.S. retailers like Wal-Mart, Target, Best Buy began offering some of the season's best online deals, which were traditionally reserved for the Monday after Thanksgiving, several weeks in advance.

"Consumers are not holding back their purchases for deals on certain days anymore and that trend is becoming increasingly clear," said Jay Henderson, strategy program director at IBM.

Early discounts also hurt in-store sales during the Thanksgiving weekend which fell 11 percent, according to the National Retail Federation.

Cyber Monday sales continued to be driven by mobile traffic which grew 38.3 percent this year, even as the average order value remained flat at $131.66, IBM said.

(Reporting by Nandita Bose in New York; editing by Andrew Hay)