Gift buyers are expected to spend £4,000 every second
Amazon has said business has been "extremely busy" on the day when online retailers are expected to take a record amount of sales.
The first Monday in December is considered the busiest shopping day of the year, and one-day sales are expected to top £350m.
About £320m was spent on the busiest day last year.
But online spending still accounts for only 4% of total retail sales, according to national statistics.
BBC technology correspondent Rory Cellan-Jones said as Britain struggled to emerge from recession, retailers were desperate for evidence of a surge in spending this Christmas.
"We've seen a high number of orders for products like Susan Boyle's album and the new Harry Potter DVD," a spokesman for Amazon said.
However, it was too early to say whether sales would top the £1.4m it took on the equivalent day last year, he added.
And retailer John Lewis said that by 1800 GMT, page views for its website were already 20% up on the entire day a year ago.
But as shoppers flex their credit cards on what has been called "Cyber Monday", consumers have been warned they are more likely than ever to become the victims of fraud and fake websites.
Tony Neate, of Get Safe Online, said shoppers should scrutinise websites very carefully.
And in May this year, the Office of Fair Trading said almost a third of internet users were not shopping online because they did not trust the system.
Personal security was high on the list of concerns.
Last year, an estimated 29 million people used the internet to buy Christmas presents, and forecasts suggested 5.2 million did so at work.
Overall retail sales last year were predicted to reach £13.6bn ($19.9bn), according to the internet trade body IMRG.