For the period up to midday, the average footfall across the UK was 4.2% lower than for the same hours on Boxing
Day last year, according to retail intelligence experts Springboard.
The picture was much rosier for stores in London, which enjoyed a 15% increase in footfall.
Shoppers visiting the West End are expected to spend around £50m in the Boxing Day sales, according to the New West End Company, which represents more than 600 businesses in Bond Street, Oxford Street and Regent Street.
Chief executive Jace Tyrrell said the West End was enjoying a "Boxing Day bounce" with international tourists "out in force driven by the weaker pound".
According to Barclaycard, 26 December is the most popular day to shop during the Christmas sales period, with three in 10 people planning to take advantage of the deals on offer.
Bargain hunters rose early to queue outside stores as the post-Christmas sales saw prices slashed by as much as 70%.
Shoppers queued outside stores around the country before first light as stores swapped their festive window displays for huge signs promising hefty discounts.
People were pictured inside shops with armfuls of half price clothes, with more than half a million shoppers are expected to descend on central London.
Michael Ward, managing director of Harrods, said the store's butlers served customers hot beverages as they queued outside the building before the sales began.
He told Sky News: "We've already got queues outside every aspect of the building - people have been here since seven o'clock.
"Our butlers are just out serving them with teas and coffees - so we're going to have a very civilised but very good sale."
He said consumers are attracted to the department store on Boxing Day because they offer the "exceptional" and "special".
Boxing Day still remains a key shopping day, with twice as much money spent on this day last year than Black Friday.
The period between Christmas Day and New Year generated £12bn in sales, according to ShopperTrak.
But retail experts Springboard said the decline in footfall on Boxing Day on three consecutive years indicates the lessening in importance of Boxing Day as a trading day.
They said footfall up to midday was 10% lower than on 22 December, which was the peak trading day before Christmas this year, and 9.4% lower than on Black Friday.
It has been a torrid year for retailers, with notable high street names such as Poundworld and Maplin falling into administration, Marks and Spencer and Debenhams announcing plans to shut stores, while Superdry, Carpetright and Card Factory issued profit warnings.
Online fashion retailer ASOS also issued a profit warning on the back of a "significant deterioration" in sales growth.
It is the latest big name to warn of the effects of weaker consumer confidence in the run-up to Brexit.
But its statement also signalled there were bargains to be had for price-savvy consumers.
While the sales will start in most shops on Thursday, John Lewis started its clearance on its website at 5pm on Christmas Eve. Argos also started its sale online on Christmas Day, as it did last year.
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