Image copyright Getty Images Image caption Amazon employed singer Ariana 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? said that the goods are often cheaper at other times of the year.

However Amazon said its website was transparent, and consumers could compare price changes over time.

The 36 hour sale - aimed at subscribers to the Prime shopping service - offers 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 offers start at midday UK time on Monday, and are co-ordinated across the globe.

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."

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What Amazon Prime customers say:

Jonny Grant, an IT consultant from London, found that the price of a computer monitor on Amazon Prime actually went up at midday on Tuesday, from £299 to £354.

"I've been caught out," he told the BBC. "I didn't buy it, because I wouldn't want to reward them for this kind of price hike."

But Greg Hine, from Edinburgh University, said he'd found Prime Day to be fairly reasonable. "Every year I get my protein powder for £29.99 instead of £48. I have to say Prime Day works for me."

Several websites track the prices on Amazon. One, called Lovethesales.com, agreed that some so-called bargains were not all they were cracked up to be.

It said a Sony Playstation was currently being advertised with 14% off, at a price of £299.

However on 28 November last year the same console was £40 cheaper.

'Bombardment'

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."

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In response, Amazon said customers could see for themselves whether they were getting a bargain.

"One of the great things about shopping online is that customers can quickly and easily compare prices," the company said.

"For many of our deals the new price and the previous Amazon price can be seen at the product detail page so customers can make an informed decision."

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.