Christmas Day is now a "fairly significant" event in the online shopping calendar as people redeem gift cards and hit the sales early, IMRG (Interactive Media in Retail Group) said.
Boxing Day is also estimated to get close to being a billion-pound online shopping day, with sales forecast to reach £984 million, up 15% on last year, according to IMRG and SimilarWeb data.
Justin Opie, managing director at IMRG, said: "It might not seem an obvious fixture in the retail calendar, but Christmas Day has actually become a fairly significant online shopping day in the UK.
"Whereas gifts were traditionally physical items, many people now receive gift cards which they can instantly redeem on the day to download the kinds of items that have become highly virtual - games, music, in-play app upgrades etc.
"Many retailers will also have set their post-Christmas sales campaigns live on their sites by then, which are very easy to browse using the kinds of devices many people have access to - smartphones, tablets etc - during a quiet digestive moment post-Christmas dinner perhaps."
Nitzan Tamari, vice president of marketing at SimilarWeb, said: "Last year, both Christmas and Boxing Day were big days in terms of desktop and mobile web traffic in the UK. We saw over 73 million visits to the category on Christmas Day alone.
"We're waiting to see if Boxing Day beats Black Friday this year - these were the biggest shopping days in 2015 and last year we saw Black Friday win out by just under three million visits."