Headphones on Amazon from some third-party sellers had misleading reviews for different products like cuddly toys and umbrellas, Which? found.
In an investigation, the consumer watchdog discovered that nine out of 10 of the top-rated headphones on the site earlier this year had glowing reviews for a range of unrelated products.
Amazon said that it had taken action on the listings and sellers in question.
The UK government is investigating the issue of fake reviews.
Reviews on products sold on Amazon and other online marketplaces play a big part in persuading consumers to buy one item over another.
Which? found that some sellers were using loopholes in review merging, which can artificially inflate how high up in listings products are seen by customers.
Merging reviews on Amazon can be legitimate, and is useful for sellers who want to manage a catalogue of very similar products – such as the same product but in a different colour.
But using such reviews for unrelated products is against Amazon’s terms and conditions, because it can make something look more popular than it is.
Which? focused on just one category – Bluetooth-enabled headphones – and followed the reviews for the top 10 products over the course of a month, from February to March this year.
Most of the brands were not household names and were all sold by more than one seller, so Which? was unable to determine whether the brands themselves were implicated in any wrongdoing.
None of the sellers contacted by Which? replied to its requests for comment.
The most highly-rated product boasted five stars, carried an “Amazon’s Choice” stamp of approval and came with 40 glowing reviews. But none of the reviews actually related to the headphones themselves – instead they were for cuddly toys.
Another headphone listing had 863 reviews for a personalised jigsaw puzzle, while a third had 1,386 reviews for beach umbrellas
Next to the other products on the list, Which? found more completely unrelated reviews, for souvenir mugs, razor blades, and an extension cable.
Some, but not all of the reviews, came with pictures of the other products.
Only one of the headphones on the list, made by one of the best-known audio electronics firm Bose, showed no evidence of review merging. But its headphones were ranked only eighth best out of the 10 investigated.
Which? director of policy Rocio Concha said: “Unscrupulous businesses are exploiting weaknesses with Amazon’s review system, leaving shoppers at risk of buying products boosted by thousands of bogus five-star reviews.”
It was, she added, more evidence that the Competition and Markets Authority’s ongoing fake reviews investigation was vitally needed to “protect people from these unfair practices”.
In response, Amazon said: “We have now taken appropriate enforcement action against the product listings and sellers in question.
“We have clear guardrails in place to prevent products from being incorrectly grouped, either due to human error or abuse.
“Our proactive measures detect and block the vast majority of abuse in our store automatically: however, we are disappointed when bad actors evade our system and we will continue to innovate and invest in our tools and processes.”
Although Which? only looked at headphones, it said it had also seen the issue across other categories, such as smartphone chargers.
And the watchdog also found evidence of review hijacking, where sellers have their reviews stolen by others.
Doing this is relatively easy to do, experts told Which? Sellers simply find dormant listings and retop them with their own product name.
Which? was first alerted to the issue by buyer Alistair Soames, who complained to the watchdog that he had been shopping for earbuds on Amazon, and noticed that some of the reviews for them were for shower curtains and kitchen knives.
“I read the reviews and many were absolute nonsense. It was the pictures that gave it away – I saw a shower curtain and thought ‘hang on a second, what has that got to do with headphones?'”