The websites of some of the UK's biggest retailers have been struggling to cope with the volume of traffic which is set to make this year's Black Friday the biggest online shopping day in UK history.
By the end of the day retail analyst Experian-IMRG predicts that spending online will hit £1.07 billion, a 32% increase on last year's £810 million, and companies such as John Lewis and Argos have struggled to keep their websites up and running, the Press Association reports.
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.
One IT analyst estimated that the company could have lost £2.8 million.
Danny Quilton, chief technology officer at Capacitas, said: "This down time is undoubtedly expensive for John Lewis. They report annual online revenues of £1.4 billion and here at Capacitas we typically see 2% of demand taking place on Black Friday.
"In addition 10% of the Black Friday day's sales take place in the peak hour. This means an hour of downtime on the John Lewis website could amount to £2.8 million in lost revenue."
The retailer said: "There are record levels of demand for our website today and for some people it is taking longer than normal to shop on johnlewis.com. We apologise to customers for any inconvenience caused."
The Argos website also crashed several times earlier on Friday 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.
Frustrated shoppers complained about the Argos website, with one saying on Twitter: "10 min queue online, then it told me 8 days for delivery with no option for 24 hour, and then it crashed."
Another frustrated user tweeted: "shouldn't advertise doing Black Friday if your website can't handle it. Trying to pay and keeps crashing".
Argos responded to customer concerns by saying on Twitter: "Order confirmations are taking longer than normal. Please sit tight, confirmation will be with you shortly."
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 #justsaying."
Other retailers being singled out on social media for slow websites include fashion hub Boohoo.com, Boots, River Island and Debenhams.
The Metropolitan Police are advising Black Friday customers to use a different password for each online retailer they visit after a man was arrested on Thursday for trying to breach Tesco's online system using usernames and passwords obtained from previous hacks.
Online retailer very.co.uk said it had more than half a million users visit its site by 9am having launched sales at midnight, with a surge in traffic at 7am when its hourly deals went live.
The company is predicting at least 250,000 orders by the end of the day with one TV being sold every six seconds and one games console every four seconds.
Currys PC World also reported its biggest-ever start to Black Friday with eight sales per second and 100 large screen TVs selling per minute online.
The electrical retailer had 400,000 visitors per hour to its website during peak trading, a figure up 70% on last year.
Ebay is expecting nine million Britons to visit its website by the end of the day.
Guy Anker, managing editor of Moneysavingexpert.com, said: "The benefit of shopping online is it's so much easier to compare deals with a few clicks so they know whether the price after that 20% discount really is the cheapest on the market.
"Let's also remember today is a working day, with millions on office computers or on their smartphones at work able to shop easily and quickly online."
As a result of the surge in online sales analysts predict a total four day spend of £3.2 billion from Black Friday to Cyber Monday.
But this was not reflected on the high street and in supermarkets where there were no signs of the huge crowds which gathered last year, or the scuffles which broke out as customers fought over big-ticket items.