91Èȱ¬

Amazon has sued four firms that pay people to produce reviews that then appear on the online retailer's site.

The company alleges that the paid opinions "undermine" its review system which customers believe come from unbiased sources.

Amazon is seeking damages from the four sites and wants them to stop producing the reviews.

The four companies named in the legal complaint have not yet responded to Amazon's allegations.

"While small in number, these reviews threaten to undermine the trust that customers, and the vast majority of sellers and manufacturers, place in Amazon, thereby tarnishing Amazon's brand," said the retailer in documents filed to the court in Washington.

As well as subverting the review process, Amazon accused the sites of trademark infringement, and violations of local consumer protection statutes and cybersquatting laws that govern who can own a domain name.

It alleged that the paid reviews were being placed on its site at a slow rate designed to outwit the detection system it ran to spot such content. In its legal papers, Amazon said it verified its allegations by surreptitiously paying one of the services to write reports about products. This resulted in them getting "glowing" reviews, it said.

The legal papers name four review producers that operate via sites called buyazonreviews.com, buyamazonreviews.com, bayreviews.net and buyreviewsnow.com.

Since the legal action was filed two of the sites named have gone offline. Those still operating have yet to respond to the 91Èȱ¬'s request for comments.

Mark Collins, who runs the Buy Amazon Reviews site, defended his business in an interview with The Seattle Times. "fake" reviews but provided "unbiased and honest" opinions about products.

The sites pay between $19-$22 (£13-£15) for each review which typically result in products getting a four or five star rating. Higher star ratings can often translate into higher sales.