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The index reveals that £15.2bn was spent online in October to December alone on the run up to Christmas.
Within key sectors such as electrical goods, the research indicates that growth in online sales comes at the expense of high-street retailers.
Anthoula Madden, vice-president at Capgemini UK's consumer products and retail team, said, "Online growth has proven robust and sustainable over the past year, increasing its share of UK retail from 10p in the pound to 15p.
"While we are yet to see overall high-street sales decline there can be no doubt online is growing its share at the expense of bricks and mortar retailers, and we believe that this trend will continue."