Britain's competition watchdog on Friday asked Facebook and eBay to conduct an urgent review of their websites to stop the sale of fake and misleading online reviews.
The Competition and Markets Authority said it has found "troubling evidence" that there was a growing marketplace for misleading reviews on the two sites.
CMA said it found over 100 eBay listings offering fake reviews for sale between November and June, while it also identified 26 Facebook groups where people offered to write fake reviews or businesses recruited people to write them on popular shopping and review sites.
CMA said Facebook had informed the regulator that it had removed most of those 26 groups.
"We have zero tolerance for fake or misleading reviews. Listings such as these are strictly against our policy on illegal activity and we will act where our rules are broken," an eBay spokesperson said.
Facebook was not immediately available for comment.
Earlier this month, Facebook announced plans to create 500 new tech jobs in London by the end of the year, with many working on systems to detect and remove malicious content, fake accounts and harmful behaviour.
"Fake and misleading reviews not only lead to people making poorly informed choices and buying the wrong products, but they are also illegal under consumer protection law," CMA said in a statement.
The social network has been under scrutiny from regulators around the world over its data sharing practices as well as fake news and hate speech on its networks.
