Half of Black Friday deals were actually cheaper before and after the event, a watchdog has found.

The consumer group, Which? warned shoppers to “do your research” after it found just 8 per cent of offers were exclusive to the day, while four in 10 could be picked up cheaper after the event and one in 10 items cost less in the months before.

Which? tracked 178 deals on 20 popular gadgets and appliances at Amazon, AO, Argos, Currys and John Lewis every day for three months before and two months after Black Friday last year.

It found that only 90 out of the 178 deals were cheapest on Black Friday.

AO and Currys may also have broken advertising rules after Which? found “numerous” examples of offers that appeared to exaggerate the apparent discount by using an artificially high “was” price.

Rules say the “was” price should be the most recent price the item was sold at, for at least 28 consecutive days.

Pete Moorey, head of campaigns at Which?, 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.”

Both AO and Currys denied they had broken advertising rules.

A Currys spokesman said: “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.”

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Amazon said six of the eight products reviewed were cheapest on Black Friday and explained it spread out deals over several days in response to customer feedback.

Argos said it did not intend to mislead customers and updates its prices regularly to ensure the best deals.