Facebook and eBay have been ordered to take urgent action to stop fake online reviews that are increasingly being used by companies to deceive customers.

The Competition and Markets Authority said it had found “troubling evidence” of a thriving marketplace for fake and misleading online reviews.

After spending the past eight months looking for evidence, the CMA said it had unearthed more than 100 listings on eBay from companies and individuals offering fake reviews for sale.

The UK regulator had also identified 26 Facebook groups where people offered to write fake reviews to order, or businesses recruiting people to write fake and misleading reviews for popular shopping and review sites.

Having great online reviews can make or break a business, and companies spend huge sums creating and managing their feedback.

In October 2018, Which? said two large Facebook groups, plus some smaller groups, may between them have up to 87,000 members potentially engaged in writing fake reviews.

Andrea Coscelli, the CMA’s chief executive, said he did not believe Facebook or eBay were intentionally allowing this content to appear on their websites, but he warned they must do more to prevent it appearing.

“Lots of us rely on reviews when shopping online to decide what to buy. It is important that people are able to trust that reviews are genuine, rather than something someone has been paid to write,” Coscelli said on Friday.

“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.”

Both companies have been told to conduct an urgent review of their sites. It is estimated more than three-quarters of UK internet users consider online reviews when choosing what to buy.

Billions of pounds of consumer spending is influenced by reviews every year. The use of fake and misleading reviews not only leads to poor choices, it is also illegal under consumer protection law.

Facebook said it removed 24 of the 26 groups the CMA reported to it. “We know there is more to do which is why we’ve tripled the size of our safety and security team to 30,000,” a spokesman said.

In a statement eBay said that sellers identified by the CMA have since been suspended. “Listings such as these are strictly against our policy on illegal activity and we will act where our rules are broken.”