An investigation by the consumer group Which? found hundreds of unverified accounts left top reviews on tech products in a single day.
Which? says glowing reviews from unverified buyers are a "telltale sign of 'fake' reviews," and said its findings showed they had become a "serious problem" in online shopping.
The research found little-known brands dominated search results for several popular tech products including cameras, headphones and smart watches, often thanks to reviews regarded as suspicious by Which?.
But Amazon has said it invests "significant" resources in weeding out fake reviews, and took action against firms and individuals suspected of using them.