The growth in online sales of non-food products slowed to 7.2% in December, from a 12-month average of 10%.
In December, online sales represented 24.3% of total non-food sales in the UK, compared with 22.6% in December 2015, the BRC-KPMG Online Retail Sales Monitor shows.
This brings the 12-month average penetration rate up to 21.7%.
Over the three months to December, online sales of non-food products in the UK grew 9.5% year on year. Total non-food sales grew 1.3%.
During that period, total online sales of non-food grew by 2.9 percentage points, while in-store sales declined by 1.6 percentage points.
Helen Dickinson, chief executive of the BRC, said: “After three months of double digit growth for online sales, December saw the third slowest growth rate of 2016 at 7.2%.
“At first glance this may appear a disappointing figure; but with December taking the second highest volume of online sales in the year, after November, this makes it an extremely tough comparable period. So overall, this is a relatively solid performance.
“As with total sales, there was a shift in spending towards the end of the month compared with last year and a slow start to the festive trading period was offset by a spending spree during the Christmas week.
“Shopping online is becoming increasingly popular during the festive month. The channel won its greatest share of December sales to date, with nearly a quarter of all purchases being made online. No doubt this was partly due to customers being able to receive deliveries right up to the two days before Christmas, thanks to retailers extending their delivery guarantees this year.”