Total spending today — dubbed Manic Monday — is forecast to hit the equivalent of £22.4m(€26.3m) an hour, according to shopping comparison website Kelkoo.
The busiest day on the High Street is expected to be December 18, when 10 million consumers are predicted to make a last-minute dash to buy Christmas gifts at a total cost of £1.1 billion, or £764,000 per minute.
Visa Europe said the busiest online shopping day in the previous two years had fallen on the Monday closest to the start of December.
The payments company said it expected a total of £265m to be spent online today by consumers using a Visa card, with 80% of those using debit cards.
It predicted transactions would peak in the lunch hour between 1pm and 2pm and again at around 7pm as shoppers logged on after returning home from work.
A study by Visa had found 74% of consumers planned to buy presents online this Christmas, with 30% looking to do more shopping on the internet than last year.
Visa Europe commercial director Steve Perry said: “Shopping online is now a natural part of the gift-buying process for consumers and a key part of Christmas sales strategy for retailers. The combination of monthly salaries being paid and people taking delivery times into consideration should combine to deliver a record day for internet shopping.”
Retailer Argos also said it was predicting today to be the busiest for online delivery orders, falling after the last pay day before Christmas, but still allowing time for delivery and wrapping before Christmas.
Argos spokesman Matt Roberts said top sellers driving Manic Monday would be audio gadgets such as the iPod Touch and iPod Nano, Toy Story toys including Buzz Lightyear and Mr Potato Head and the recently released video games Call of Duty: Black Ops and Gran Turismo 5.
However, police warned that criminals were looking to cash in on the online spree.
The National Fraud Authority and the City of London Police urged consumers to take extra care when purchasing popular Christmas gifts online, with reports suggesting that gadgets like smart phones and digital cameras featured highly in fraud cases.
They urged consumers to use trusted secure websites and search online forums for feedback, check for a “real world” address and phone number, check for “https” and the padlock or similar symbols to indicate a secure connection, and to remember that legitimate popular technology and designer items were rarely discounted.
Det Chief Supt Steve Head, head of the City London Police Economic Crime Directorate, said: “Fraudsters hide behind showy websites and never have to face their victims, but we have to pick up the pieces.
“This is a crime that ruins Christmas. These victims lose a lot of money, but perhaps more upsetting is that they don’t even have the gifts to give their family and friends.
“Christmas is a time of goodwill, but don’t show that generosity to the fraudsters. Do the checks and be secure.”