Retail research firm Verdict's lead analyst Patrick O'Brien said that Monday – dubbed "Mad Monday" – was going to be an "extremely busy" shopping day because consumers had come to expect last-minute sales.
This is a trend we've seen over the last few years: shoppers are leaving their shopping later and later because they've come to expect a high level of discounting in the final few days before Christmas," he told CNBC.
"It's a game of chicken between retailers and shoppers. The shoppers know they have to buy everything before Christmas – but they also know that the later they leave it, the most likely they are to cash in on discounts by retailers."
And some stores have already given in, with Gap offering an extra 20 percent off some of its sale items on Monday. Marks and Spencer, meanwhile, has cut some prices by 30 percent ahead of Christmas, and department store chain House of Fraser is offering gifts at half-price.
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Verdict expects shoppers to spend £88.4 billion over the festive period (the fourth quarter of 2013) – that's £1.95 billion more than in 2012.
iPad sold every 10 seconds
Monday's rush comes after British department store John Lewis reported record sales for the last full-week before the festive holiday – indicating that the £7 million it spent on its much-anticipated Christmas advert had paid off.
The chain – which has 40 stores across the U.K. – said sales topped £160 million for the first time ever over the week to Saturday. The total figure came in at £164.4 million – a 31.8 percent year-on-year increase.
John Lewis said it sold one iPad every 10 seconds over the week - "proving that it is once again the must-have gift for Christmas" - and Apple's iPad mini was the retailer's best-seller for the second week in a row.
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In a further indication of what some might find under the tree on Wednesday, the chain said other "hot sellers" included a red women's dressing gown, Junior Scrabble and a cocktail gift set.
John Lewis noted that the way people shopped had changed this festive period, with sales flattening out before the final week's surge. But it added that it expected the last-minute spending rush to continue on Monday and Tuesday, as people race to complete their Christmas shopping.
"It's clear that much of Britain has yet again taken shopping for Christmas right up to the wire," Paula Nickolds, buying and brand director at John Lewis, said in a statement. "Buoyed by this week's record trading, all of our channels are braced for the final three shopping days before Christmas."