Black Friday shoppers have been advised to "do your research" after a watchdog found so-called deals last year were cheaper in the months before and after the day itself.
Consumer watchdog Which? found 49% of last year's deals were not the cheapest on the day, with only 8% being one-day only offers.
Black Friday falls on November 25 this year.
The watchdog tracked 178 deals on 20 popular types of technology and home appliances in the three months leading up to the day, and two months afterwards.
Comparing the cost of products from Amazon, AO, Argos, Currys and John Lewis, they found one in ten (12%) were cheaper in the months leading up to Black Friday, with four in ten (38%) cheaper in the several weeks after.
Only 90 products were found to be cheaper on Black Friday.
Which? said they believed Currys may have broken advertising rules in relation to "numerous" examples of offers that appeared the inflate the 'was' price - to make deals look more substantial than they were.
The watchdog's 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."