Online sales grew 16% year-on-year in August to £3.8bn, according to the IMRG Capgemini e-Retail Sales Index.
The figure was a 10% drop on July's sales, which the companies behind the index say is in line with seasonal trends seen in previous years.
The key drivers of the annual growth are the clothing, accessories and electricals sectors.
Capgemini said many retailers in these sectors are working to improve their online businesses with better visuals, more convenient delivery and increasing use of Web 2.0 tools.
Mike Petevinos, head of consulting for retail at Capgemini UK, said, "The key drivers of this growth are clothing, accessories and electricals. Consumers continue to see the benefits of greater choice, price competitiveness and convenience from buying online in these sectors."
Online retail sales have performed well all year, despite a difficult few months for the high street.
Tina Spooner, director of information at IMRG, said, "The continued annual growth in the online retail market is evidence that this medium is withstanding the challenges of the economic downturn and the retailers that continue to expand and improve their online presence will no doubt reap the benefits during the festive trading period."