According to independent research group Which?, the online retail giant has had a host of fake five-star reviews uploaded to products from unfamiliar brands, meaning these mystery products climb higher up the search results.
The probe found that the top-rated reviews for items such as headphones, smart watches and fitness trackers were dominated by unknown brands, and thousands of the reviews were unverified, meaning there was no evidence the reviewer bought the product.
Amazon has said it uses automated technology to weed out false reviews, and has invested "significant resources" to protect its review system "because we know customers value the insights and experiences shared by fellow shoppers".
It added to the BBC: "Even one inauthentic review is one too many."
But Which?'s probe suggests the fake reviews are commonplace, and found that during a search for headphones, 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.
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.