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Amazon employed singer Ariane Grande to promote Prime Day
Shoppers thinking of taking part in internet giant Amazon's Prime Day are being warned that not all the items on sale could be the bargains they seem.
Consumer group Which? stated that the goods are often cheaper at other times of the year.
Amazon Prime Dayaimed at subscribers to the Prime shopping serviceoffers discounts on a range of goods.
The deals are time-limited, with shoppers being told that some items are only available while stocks last.
The heavily-promoted sale is due to last for 36 hours across the world, starting at midday UK time.
However, Which? warned that some goods could be bought more cheaply outside the promotion period.
"Although these time-limited sales events can offer great discounts, not all offers will be as good as they seem," said Adam French, Which? consumer rights editor.
"It can be easy to get swept along by the hype and excitement on the day, so we recommend preparing in advance and researching what you want to buy, to make sure you can tell the difference between a good deal and a dud on Amazon Prime Day."
Other consumer groups questioned whether targeted sales messages on people's mobile phones should be allowed to continue unabated.
"There's so much data. Retailers can work out so much about us," said James Daley, the founder of Fairer Finance.
"It turns into a bombardment of high-pressure sales messages that can lead people to make the wrong decisions. It's time for us to have a debate on whether more boundaries need to be drawn."
The launch of Amazon Prime Day is accompanied by a big marketing budget.
Pop band Take That performed at an "Unboxing Amazon Prime Day" event, while Ariane Grande opened a promotional concert in New York.
Several British newspapers and websites also provided readers with direct online links to the shopping site.
Many of them stand to make a small amount of money on each sale.