Ossie Bayram, commercial director at PriceSpy, said: “While there are deals to be had, never assume you're getting a true bargain just because it's advertised as a Black Friday exclusive.
“Do your homework and check out what the product cost in the weeks running up to the sales, because if it was cheaper before, it'll likely be cheaper in the future too."
Among the products highlighted by Which? as having been listed for the same price or less as on Black Friday was a set of Bose headphones listed for £249 at Currys, but available cheaper or for the same price on at least 15 occasions in the six months before.
A high-end De’Longhi coffee machine at John Lewis was also listed for £1,285 last Black Friday but it would have cost less than £1,200 on several days in May and June last year, Which? said.
Currys said that 45 per cent of its deals this year are at their lowest price, but that it has other sales and price-matches, while John Lewis said its “never knowingly undersold” guarantee means it matches prices year-round, although this is under review.
AO.com said it offers “great deals” for customers all year round and Argos said hundreds of its products were at their lowest ever price on Black Friday.
Amazon said it also offers low prices “all year round” with a number of “fantastic seasonal deal events”.
Retailers are expecting online sales to soar this year with physical shops forced to close by coronavirus restrictions.
Meanwhile, the National Cyber Security Centre has issued a warning for bargain-hunters to take extra care as criminals are likely to target online shoppers.
It warned that the festive period can be a lucrative time for scammers as online transactions increase.
Nearly nine in 10 Black Friday “deals” aren't real discounts, Which? has found, as its investigation suggests shops put up prices ahead of the big day.
The consumer group said shoppers should do their research before splashing out in this week’s sales as the so-called discounts might actually be bigger before or after the day itself.
The organisation tracked the prices of 219 products advertised as “Black Friday deals” last year at Amazon, AO.com, Argos, Currys PC World and John Lewis and found that 85 per cent were actually cheaper of the same price in the six months before. Many were also cheaper in the days and months after.
While it is too early to tell what the best-sellers on Black Friday will be this year, with the event taking palace on November 27, data from PriceSpy showed that the average cost of laptops and tablets has been creeping up over the past two months ahead of the sales.
Natalie Hitchins, head of home products at Which?, said people may be more tempted to browse for bargains this year as they will be stuck at home.
But she added: “Deals that look too good to be true often are, so don’t fall for time-limited offers and if you are looking for something in particular, do some research first. That way you’ll know a genuine bargain when you see one.”