Amazon has filed a lawsuit against four companies that pay people to write product reviews that appear on its website, according to Digital Spy.

The online retailer claims that the practice undermines its reviews system and misleads customers into thinking they are from an impartial source.

"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 Amazon in a statement.

The firm is seeking damages from the four companies it has targeted - buyazonreviews.com, buyamazonreviews.com, bayreviews.net and buyreviewsnow.com - and wants them to stop commissioning reviews for its website.

The e-commerce giant is also accusing the firms of trademark infringement, violating cyber-squatting regulations, and using underhand methods to sidestep its detection system.

Paid-for reviews typically rate a product at either four or five stars, sometimes bumping up its overall rating and helping boost sales.

Since the lawsuit was filed last week, bayreviews.net and buyreviewsnow.com have gone offline.