In a letter to marketing departments, marketing agencies and their clients, the CMA said that it has concerns about the labelling of paid promotions and the use of fake reviews.
The CMA has been investigating the use of online reviews and endorsements to ensure they are in line with consumer protection law. These "have become part of everyday life and significantly influence people's buying decisions", it said.
The use of editorial content "to promote a product where a trader has paid (financially or otherwise) for the promotion, without making this clear to the consumer, is unlawful and may lead to enforcement action", the letter said.
Businesses that pay for promotions in articles and blogs are responsible for ensuring that this has been made clear to readers or viewers, the CMA said. These businesses should instruct publications and bloggers to identify the promotions clearly, and turn down any requests for promotions that are not clearly labelled.
Writing or commissioning a fake review is a breach of consumer protection law and may lead to civil or even criminal action, the CMA said.
Business should therefore not offer inducements to customers to write positive reviews, and should not write fake reviews about their own or competitors'' products, it said.
It a related letter to online publishers, the CMA said that "everyone involved in online endorsements is responsible for ensuring that paid promotions are clearly labelled or identifiable as paid-for content. This includes bloggers, vloggers, micro bloggers (who comment on social media, including celebrity tweeters), and online newspapers and magazines".
If publishers or bloggers are paid to feature products, whether financially or otherwise, this must be clearly labelled or they too could face civil or criminal action, the letter said.
"Misleading readers or viewers may not only damage your reputation – it also falls foul of consumer protection law and could result in enforcement by either the CMA or Trading Standards Services," the CMA said.
In both letters the CMA said that it has recently investigated three cases where businesses had failed "to clearly identify where they had paid bloggers or online publications to feature particular products in their blogs and articles".
The letters reaffirm the need for businesses to be "honest, transparent and open", said copy clearance expert George Campbell of Pinsent Masons, the law firm behind Out-Law.com.
"The CMA is emphasising the need for businesses to be aware of their obligations and in particular that marketing materials should be clear and not misleading. Failing to meet these requirements can lead to bad publicity and reputational damage, as well as potential action by enforcement authorities," he said.
The Internet Advertising Bureau issued guidelines last year on 'native advertising' and said that businesses should ensure has prominent 'visible visual clues' that allow consumers to identify it as marketing material.