Black Friday sales offer few real deals with most goods cheaper or available for the same price at other times, according to consumer group Which?.
It found that just four of 83 products they studied last year were cheaper during the Black Friday promotion.
Among products cheaper outside of Black Friday were a tumble dryer, smart speaker, coffee maker and TV soundbar.
Which? home products and services chief Natalie Hitchins said shoppers should do research and never impulse buy,
The consumer group tracked the products on Black Friday last year - 23 November. The items, from retailers including Currys PC World, Amazon, and John Lewis, were monitored six months before the date and six months after.
Just four products were cheaper on Black Friday than at other times of the year. Six in 10 items were cheaper or the same price on at least one day in the six months prior to last year's Black Friday event.
When looking at the two-week period surrounding Black Friday itself - including sale prices in the week before and after - it was found that three quarters of products were cheaper or the same price in the six months after.
- A Samsung soundbar that Currys PC World offered at £299 on Black Friday, but which within a month dropped by £49 for at least five days
- A De'Longhi coffee machine priced at £399 by John Lewis on Black Friday. It was cheaper on at least 64 days in the six months after
- An Amazon Echo on offer at £54.99 on Amazon, but was cheaper on at least 13 occasions before Black Friday
- An Indesit tumble dryer from Currys PC World was £20 cheaper than its sale price of £199 just two weeks before Black Friday.
While Which? did not find evidence that any of the retailers were breaking the law, the consumer group said it showed that so-called deals may not be all they are claimed.
Ms Hitchins said: "We have repeatedly shown that deals touted by retailers on Black Friday are not as good as they seem. Time-limited sales can be a good opportunity to bag a bargain, but don't fall for the pressure tactics around Black Friday.
"Our investigation indicates that this popular shopping event is all hype and there are few genuine discounts," she said.
Retailers rejected suggestions consumers were being misled and said that shoppers were getting some of the best deals of the year.
Amazon said: ""The claim from Which? with regard to Echo is false and we have made this clear in our response to them. Amazon.co.uk customers were not able to buy the Echo (2nd gen) device cheaper before Black Friday 2018."
It added that its Black Friday sale was about "thousands of deals on a huge selection of products from every category across the site, at a time of year when we know saving money is important to our customers".
Currys PC World told Which?: "Our customers tell us that they appreciate the increased choice during Black Friday where we have the most deals on at once. When we launched our Black Friday event last year, 40% of those products were the lowest price they had ever been."
And John Lewis said: "In addition to the variety of offers we have in-store and online during the Black Friday period, our commitment to being 'Never Knowingly Undersold' means that we continuously monitor and match the prices of our high street competitors throughout the year."
Which?'s advice for consumers during this year's sales bonanza is:
- Don't impulse buy. Work out what you need or want to buy and how much you're prepared to spend before you start shopping in the sales
- Don't panic. Don't worry about missing out on a good offer as Which? research has revealed that you could be better off waiting until after Black Friday anyway
- Find out about offers in advance. Sign up to your favourite retailers' newsletters for inside tips, or check their social media pages ahead of the sales for any clues or hints to what will be on offer
- Look at the price on previous days, using websites such as PriceRunner, to make sure you really are getting a good deal
- Don't always trust reviews you find online as a recent Which? investigation found shoppers risk being misled by fake online product reviews.