The Competition & Markets Authority has urged Facebook and eBay to urgently stop the sale of fake reviews through their sites.
The move follows an investigation in which the UK’s competition watchdog found "troubling evidence" of a thriving marketplace for fake and misleading online reviews.
Between November 2018 and June 2019, the CMA found more than 100 eBay listings selling fake reviews. It also found 26 Facebook groups where people offered to write fake reviews or businesses recruited people to write fake or misleading reviews on popular shopping or review sites.
Facebook and eBay have each told the CMA that they will co-operate, the watchdog said. Facebook said most of the 26 groups found have been removed after it was notified by the CMA.
An eBay spokesman said: "’We have zero tolerance for fake or misleading reviews. The sellers we have identified from the CMA report have been suspended and their listings removed. Listings such as these are strictly against our policy on illegal activity and we will act where our rules are broken. We welcome the report from the CMA and will work closely with them in reviewing its findings."
The CMA estimates that more than three-quarters of UK internet users consider online reviews when choosing what to buy and that ecommerce platforms should put measures in place to ensure fake reviews do not appear. It is estimated that £23bn a year of UK consumer spending is potentially influenced by online reviews.
An investigation in April by Which? found thousands of fake and unverified reviews on Amazon. Headphones are by far the worst offender for being targeted by fake reviews for unknown brands, the consumer products review magazine found.
Andrea Coscelli, the CMA’s chief executive, said: "It is important that people are able to trust that reviews are genuine, rather than something someone has been paid to write. Fake reviews mean that people might make the wrong choice and end up with a product or service that’s not right for them. They’re also unfair to businesses who do the right thing.
"We want Facebook and eBay to conduct an urgent review of their sites to prevent fake and misleading online reviews from being bought and sold."
Editor's note: This article was updated at 8.45am on 26 June to include eBay's statement