Nine out of 10 Black Friday deals are the same price or cheaper in the six months before the day, according to an investigation.

The consumer group Which? looked at 201 of last year's Black Friday deals at six retailers - Amazon, AO, Argos, Currys, John Lewis, and Richer Sounds.

They recorded the prices every day in the six months before and after Black Friday, and on the day itself.

They found that 184 products - 92% - were the same price or cheaper in the six months before the day, while 98.5% were cheaper or the same price at some point in the six months afterwards.

Among the items were:

• A Zanussi ZWF81441W washing machine at John Lewis - £309 on Black Friday but £249 five months earlier and £289 within a month after the day

• A Bush BRC100DHEB 100cm dual fuel range cooker from Argos - £449.99 on Black Friday but the same price 66 times before then and 19 times after

The investigation also found that many shoppers do not research the products they are going to buy before they head to the shops.

This included a third of those buying baby and child products, 26% buying home appliances, and 22% buying tech products.

Ele Clark, Which? retail editor, said: "Take time to identify the products you really want and check that the 'deal' you're seeing represents a genuine saving.

"That way, you can beat the hype and be confident that you'll emerge from the Black Friday sales with quality products that will last for years to come - and all for a bargain price."

Black Friday is on the first Friday after US Thanksgiving - 26 November this year - and is known for scenes of chaos in American shops as people fight for bargains.

The retailers responded:

• Amazon: "We seek to offer our customers great value thanks to low prices all year round as well as a number of fantastic seasonal deals events.

"Our Black Friday sale is 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. And the best thing about shopping online is that customers can easily compare prices, allowing them to make an informed purchase decision."

• AO: "Customers can get deals all year round at AO and Black Friday is the biggest opportunity to get genuine, great-value deals alongside our year-round price match guarantee."

• Argos: "We're committed to offering our customers a wide range of products at great prices throughout the year and our Black Friday deals are no exception."

• Currys: "In 2020 over 98% of our Black Friday promoted products were equal to or at their cheapest price from the previous six months and 54% of our products were at their lowest ever price.

"Black Friday represents one of the best times for customers to shop with us over the year. With our unique price promise, we won't be beaten on price. Full stop. This applies even during the Black Friday period."

• John Lewis: "As a participating retailer in Black Friday, we offer fantastic deals across technology, home, beauty and fashion - these are just some of the many promotions our customers can take advantage of throughout the year."

• Richer Sounds: "Prices fluctuate pretty wildly in our industry, both up and down for all sorts of reasons, but primarily availability from suppliers and the prices we have to pay to our suppliers.

"So, this can mean that products (annoyingly for all) may be sold in the following Black Friday promotion at prices that are higher than during the preceding year but rest assured that all the deals that we sell are at the keenest prices possible at the time of the Black Friday 'event' (which, please note, starts several days before the day itself)."