Cold weather boosts online sales
It is expected to be the busiest day of the year for internet sales, with spending boosted by the cold weather.
UK consumers are expected to part with about £500m online over 24 hours.
The snow appears to have increased sales - the retailer John Lewis said spending on its online site increased by 47% last week.
However there is a warning that some traditional retailers are struggling and there could be a wave of administrations in the new year.
On the back of weak retail figures, business services firm, RSM Tenon, said the increase in VAT to 20% at the beginning of the year could prove a major difficulty for some companies.
Tom MacLennan, head of lender services at RSM Tenon, said: "Retailers want to capitalise on the Christmas period as they know that everyone traditionally spends at Christmas time and tightens their belts in January.
"With the increase in VAT to hit early next year as well, there will be more tightening of belts than before."
What is called "Mega Monday" falls on the first Monday in December.
The industry body IMRG said it expected sales to reach £500m over a 24-hour period.
The peak is reached at lunchtime when around £23m will be spent online by people taking advantage of their break to do a spot of shopping.
The UK's biggest online retailer, Amazon, said it expected to make about 23 sales every second.
The company's Gourock depot is the size of four football pitches and handles CDs, DVDs and books.
An army of "pickers" trundle up and down vast corridors holding hand-sized computers containing a list of orders. These "pickers" walk about five miles a day.
The goods are spread out in a random fashion which, the company said, makes sense.
Sandy Davidson general manager of Amazon's Gourock depot said: "You can't forecast what each customer is going to buy - so the random policy becomes advantageous in terms of smoothing the pick-path for our pickers.
"Also when you have a big seller like Toy Story 3 this year - if they're all in one place every picker in the building descends on it and you get a bit of a jam."
The peak for more traditional retailers comes much later, around 18 December.
The early online surge, with the extra boost from the snow, has led to concerns that traditional retailers will suffer this year.
Silverburn Shopping Centre on the outskirts of Glasgow said it expected sales to hold up.
David Pierotti, centre director, said: "I think with fashion products in particular, our customers prefer to check online and then actually feel Christmas and the product.
"They also like to see the grotto, they like to see the decorations, they like to get the atmosphere of Christmas which is something you don't get online."
The accountancy firm Deloitte said it expected overall sales to rise by just 1% over this Christmas period, with online sale up by 15%.
However Colin Jeffrey, a retail analyst with the firm, said some people would always prefer a physical shopping trip.
"Interestingly when you look at some of the younger demographics in our research we find that the 16 - 24 year olds still prefer to go shopping as an activity," he said.
He added: "I think retailers are being forced to look at how they use their stores differently. Perhaps in some instances more like a showroom where they can show their wares and then allow their customers to order online so they have the convenience of a home delivery. "