Top-rated reviews on popular items such as headphones, smart watches and fitness trackers were dominated by unknown brands, its probe concluded.
Thousands of these were unverified, meaning there was no evidence the reviewer bought the product, it said.Amazon said it was using automated technology to weed out false reviews.
It said it invested "significant resources" to protect its review system "because we know customers value the insights and experiences shared by fellow shoppers".
"Even one inauthentic review is one too many," it added.
But Which?'s probe suggested fake reviews were commonplace..
When it searched for headphones, it found all the products on the first page of results were from unknown brands - which it defines as ones its experts have never heard of - rather than known brands, which it defines as household names.
Of 12,000 reviews for these, the majority (87%) were from unverified purchases.One example, a set of headphones by an unknown brand called Celebrat, had 439 reviews, all of which were five-star, unverified and were posted on the same day, suggesting they had been automated.
Celebrat could not be reached for comment.
How to spot a fake review
- Do not rely on ratings - delve deeper and read the reviews
- Check the dates - look at when the reviews were posted. If many of them were posted in a short time period, it's likely they have been computer generated and are fake
- Filter reviews to remove unverified reviews. Only reviews marked as verified are those that Amazon can confirm were purchased on its website
- If products have hundreds or thousands of largely positive reviews be wary
Review Meta, a US-based website that analyses online reviews, said it was shocked at the scale of the unverified reviews, saying they were "obvious and easy to prevent".
The popularity of online review sites mean they are increasingly relied on by both businesses and their customers, with the government's Competition and Markets Authority estimating such reviews potentially influence £23bn of UK customer spending every year.
Which? Says its findings mean that customers should take reviews with "a pinch of salt".
"Look to independent and trustworthy sources when researching a purchase," says Which? head of home products Natalie Hitchins.