Black Friday shoppers have been warned to "do your research" after a watchdog found half of last year's so-called deals were cheaper in the months before and after the event.
Which? found that 49% of last year's deals for the event – which falls on November 25 this year – were not the cheapest on the day itself.
Just 8% of the deals were one-day only offers where the Black Friday price was cheaper than on any other day.
About one in ten (12%) were cheaper at some point in the three months leading up to Black Friday and four in 10 (38%) were cheaper in the weeks after Black Friday.
The watchdog tracked 178 deals on 20 popular types of technology gadgets and home appliances at Amazon, AO, Argos, Currys and John Lewis every day for three months before and two months after Black Friday.
It found that only 90 out of the 178 deals were cheapest on Black Friday.
Which? said it also believed AO and Currys could have broken advertising rules in relation to "numerous" examples of offers that appeared to inflate the 'was' price to make deals look more substantial than they were.
Rules say the 'was" price should be the most recent price the item was sold at for at least 28 consecutive days.
Which? head of campaigns Pete Moorey said: "Shoppers might be surprised to learn that only half of Black Friday deals are actually cheapest on Black Friday.
"If you're thinking about starting your Christmas shopping around Black Friday, do your research as some 'deals' may not be all they're cracked up to be."
Currys disputed the Which? findings, saying: "We are proud to offer our customers fantastic deals all year round, with prices that are truly transparent and backed by our price promise.
"We fundamentally disagree with the approach taken by Which? in this report and comply fully with the government's Pricing Practices Guidance, displaying a clear date from when the 'was' price was taken, allowing customers to make a fully informed decision."
A John Lewis spokeswoman attributed its prices to its 'Never Knowingly Undersold' promise, and said: "Our never knowingly undersold commitment means that we will match our competitors' deals and offers for as long as they run.
"We're looking forward to sharing our Black Friday deals with customers this year."
Argos said: "We offer customers thousands of great deals over the Black Friday period and work hard to ensure that all of our offers are fully compliant with the full range of regulations and guidance in pricing practice.
"We certainly do not intend to mislead in anyway. We update our pricing frequently to ensure that we can offer our customers the best deals."
Amazon said: "Six of the eight products that Which? reviewed on Amazon.co.uk last year had our lowest price on Black Friday and, in response to customer feedback, we spread out great deals over several days.
"This year, our Black Friday sale runs for 12 days from November 14 to November 25, featuring thousands of deals and millions of pounds worth of savings for customers."
AO said: "The internet is the most transparent market place ever created for consumers to be able to compare price and service associated with it and we work hard to be exceptional on both fronts. This has been recognised many times by Which? themselves.
"The sheer scale of Black Friday's great offers means that these are planned up to 12 months in advance to maximise the value and service for our customers.
"That demands significant planning for a very fluid trading environment across hundreds of offers and we are always focused on offering the best price match promise all year and especially around Black Friday."