The competition watchdog has opened an investigation into a "number of companies" that may have paid for endorsements in blogs and online articles without telling readers.
The Competition and Markets Authority (CMA) declined to name the businesses under investigation but said its concerns related to possible payments for favourable media coverage.
The investigation is the result of the CMA's call for information on online reviews and endorsements earlier this year.
In a report published today, the CMA said it had heard of instances of fake positive or negative reviews, negative reviews not being published and businesses paying for media endorsements.
But it found that most buyers who used online reviews found that the product or service met their expectations, and a spokesman said the CMA had found nothing to warrant further investigation into them specifically.
The CMA said it estimated that more than half of UK adults (54%) use online reviews and 6% use blogs or vlogs to help them decide whether to buy a product or service.
It believes £23 billion a year of UK consumer spending is potentially influenced by online reviews.
CMA consumer senior director Nisha Arora said: "Millions of people look at online reviews and endorsements before making decisions such as where to stay on holiday or which plumber to use.
"We have found that consumers who use online reviews and endorsements find them valuable, but we have also heard about some practices that may be unlawful.
"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."
A spokesman for consumer group Which? said: " The CMA was right to launch this investigation as consumers rely on reviews to make decisions, so it is critical that they are genuine.
"Retailers should be vigilant and try to root out bad practice on their sites, and consumers should check a number of difference sources, including genuinely independent experts like Which?."