The figures, from the IMRG Capgemini e-Retail Sales Index, also revealed that September is the third consecutive month where year-on-year growth was below 20%.
Mike Petevinos, head of consulting for retail at Capgemini UK, said: "The latest figures show that e-retail continues to see growth although at a markedly slower rate than we've been used to.
"While retailers continue to look to e-retail for growth, today's challenging economic climate is clearly having an effect."
However, UK online shoppers spent 24% more on clothing in September compared to August, taking advantage of early winter fashion deals as temperatures dropped.
They also spent more on electrical items, gifts and alcohol during the month.
Jo Evans, managing director of IMRG, said: "For shoppers looking to make the most out of their squeezed spending resources, buying online gives them the opportunity to search and compare products and prices easily and efficiently.
"Online retailers will need to ensure that they are prepared and ready to meet their customers' requirements by providing good product display and availability, with easy to follow payment and delivery information, in order to make the most of the current situation."
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