Black Friday bargain hunters have been warned to do their research after an investigation found many of last year's deals were cheaper in the months before and after the event.
Consumer group Which? found that 49% of last year's deals that it tracked were not the cheapest on the day.
This year Black Friday falls on 25 November - a day after Thanksgiving.
Which? found that just 90 out of the 178 deals were cheapest on Black Friday.
The group looked at 20 popular gadgets and appliances on Amazon, AO.com, Argos, Currys and John Lewis.
It said it believed AO.com and Currys may have broken advertising rules in relation to "numerous" examples of offers that appeared to inflate the "was" price to make savings appear higher than they were.
Rules require the previous price should be what the item was sold at for at least 28 consecutive days.
In one example, Which? said Currys reduced the Samsung UE40JU6740 TV by just £1 on Black Friday, even though the retailer reportedly claimed a saving of £101.
The consumer group said the TV had cost £749 since 30 October 2015, before it was reduced to £748 on 24 November 2015.
It said for 18 days in late August and early September customers could have bought it for £699.
Currys disputed the investigation's findings: "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."
Which? said AO claimed there was a £200 saving on a Vax Air Classic Pet vacuum cleaner, which was advertised for £99 on Black Friday.
It said the same model was £96.50 in the three months leading up to Black Friday in 2015 and had been sold for £69 the day before.
AO said: "We are always focused on offering the best price match promise all year and especially around Black Friday."
'Do your research'
Peter Morrey, Which? head of campaigns 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."
Black Friday began as a 24-hour sale after Thanksgiving, but like the Christmas sales period some retailers are spreading it over a longer period.
Amazon has started to offer discounts in mid-November.
The online retailer 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."
John Lewis said because of its never knowingly undersold commitment it matched its competitors' deals, while Argos said it worked hard to comply with regulations on pricing and had never knowingly misled customers.