The UK's Competition and Markets Authority is cracking down on companies publishing fake reviews or taking cash for unmarked promotional content.
The CMA said it has opened an investigation using its "consumer enforcement powers" into a number of companies over non-disclosure of paid endorsements.
The regulator said it is also concerned about businesses writing or commissioning fake positive reviews about themselves to boost their ratings on review sites.
Failure to take steps to prevent, detect, and remove fake reviews breaches the Consumer Protection from Unfair Trading Regulations guidelines, said the regulator.
As such the regulator has published guidance outlining how businesses can stay within the law, and is considering whether to invoke further "enforcement action".
It comes as Amazon has reportedly rejigged its product review systems for customers in the US, in an apparent move to mute fake ratings on the retail giant's service.
The body reckons £23bn a year in spending is influenced by online reviews.
Nisha Arora, senior director at the CMA, said: "We are committed to ensuring that consumers’ trust in these important information tools is maintained, and will take enforcement action where necessary to tackle unlawful practices."
"We have opened an investigation into businesses that may be paying for endorsements in blogs and other online articles where the payment may not have been made clear to readers. We have also published information for businesses to tell them what they need to do to help them stay within the law," she added. ®