Amazon has been accused of manipulating reviews to make sure its own brands are promoted over third party merchants, as European regulators continue to probe the retailer's practices.

Bloomberg claims it spoke to multiple merchants who had complained their product sales fell after Amazon introduced its own versions of the same products and then pushed those products up the search results.

Amazon uses a program called "Vine" which connects vendors with regular reviewers for a fee, allowing those vendors to provide their products for free for customers to review.

However, independent merchants are not able to participate in the program, meaning Amazon's own label products may be able to get more reviews on their products and so rise above the merchants' products in the search results.

According to Bloomberg, review watchdogs and excluded merchants are now "alleging Amazon is guilty of review manipulation". Amazon had said it was trying to stamp out this practice two years ago, when it intervened to stop third party merchants offering freebies in exchange for reviews.