Shoppers are expected to have spent more than £3 billion by the time Cyber Monday ends, after the American phenomenon of Black Friday led to an online surge as customers were inspired to bag bargains.
Retail giant Amazon reported record figures on Friday, with more than six million items ordered, leading to its biggest sales day ever in the UK.
While the predicted in-store chaos of last year failed to materialise, analysts have forecast that the flood of customers snapping up deals online could lead to a total four-day spend of £3.2 billion.
The sales weekend now looks to be moving towards a four-week long sales season all the way to Christmas, one expert said.
Jon Copestake, of the Economist Intelligence Unit, told the Daily Telegraph: "What we are starting to see is Black Friday, Cyber Monday and Super Saturday all roll into one in a sales season that lasts several weeks in the run-up to Christmas."
Some companies struggled to keep their websites up and running on Black Friday, traditionally held to coincide with the end of the American holiday of Thanksgiving, as huge numbers of people logged on to try to get the best deals.
John Lewis's website went down around 3.20pm and when one disgruntled customer called up to confirm an order lost online they were told the retailer was unable to take any orders and to call back in an hour.
Shoppers using the Argos website also experienced delays and consumers were confronted with a holding page saying "Oops ... Sorry to hold you up".
Page load times for the UK retailer exceeded 10 seconds, in stark contrast to the 0.5 second page load time of its competitor Amazon, and beyond the two-second threshold which some analysts say makes consumers impatient and more likely to give up on a purchase.
Multiple errors on the Tesco website were also reported, and between 10am and 11am the website slowed to an 8.4 second page load time.
Tesco denied anything was wrong saying on Twitter: "Our website is fully operational. We're very happy with our site speed. Feedback from customers today has been very positive."
Other retailers being singled out on social media for slow websites included fashion hub Boohoo.com, Boots, River Island and Debenhams.
Christopher North, managing director of Amazon.co.uk, said: "For the second year straight, customers in the UK have blown us away with their response to the many great deals on great products that we've made available for Black Friday.
"On behalf of all of us at Amazon and our many hundreds of sellers who are also offering great deals, we would like to thank our customers for choosing to shop with us and for making it a day for the record books."
The popularity of online shopping did not appear to be matched on the high street and in supermarkets where there were no signs of the huge crowds which gathered on Black Friday last year, or the scuffles which broke out as customers fought over big-ticket items.