The company, Total SEO & Marketing, has agreed to stop writing fake reviews and will take steps to remove those already posted.
It published the fake positive reviews for 86 small businesses across 26 different websites.
In addition, the CMA has written to Total SEO’s clients to warn them that third parties writing false reviews on their behalf might lead to them breaking the law. The small businesses concerned include car dealers, mechanics and landscape gardeners.
The CMA has also issued advice to businesses on how to comply with consumer protection law in relation to online reviews. It advises that PR, marketing and SEO companies should not write or arrange fake reviews on behalf of their clients and businesses should not commission third parties to write them.
This comes after the recent announcement that two websites offering a tradespeople-search service and a further three sites that carry care home reviews agreed to improve their practices for online reviews.
These cases follow an investigation launched by the CMA in June 2015 after concerns were raised not just about fake reviews, but also genuine, negative reviews not being published, as well as businesses paying for blog endorsements and other online articles.
Nisha Arora, the CMA's senior director, consumer, said: "This is our latest action to ensure that consumers can trust in the opinions that they read on online, following the CMA’s report last summer into online reviews and endorsements.
"In the next few weeks, we expect to announce the outcome of our investigation into unlabelled endorsements."