Amazon is taking legal action against four companies it has accused of deliberately flooding its shopping platform with fake reviews.
Three of the firms had nearly 350,000 reviewers on their books.
The companies act as unofficial brokers between Amazon sellers and individuals who write reviews, the tech giant says.
The reviewers get free products and a small fee in return for each review, and the firm charges the seller a fee for boosting its ratings on Amazon.
The sellers are not necessarily aware that this is being done by using fake reviews, Amazon said.
The firms it has threatened legal action against are accused of collectively targeting its platforms in the US, UK, Europe, Japan and Canada. It targeted three of them earlier this year, and has now confronted another.
One has ceased trading as a result. In a note on its website, Matronex says it has "completely shut down" following Amazon's actions.