Just Eat and the funeral provider Dignity are among the latest companies being investigated as part of an inquiry into fake online reviews.
The competition watchdog said its expanded crackdown on false and misleading information for consumers also took in the motor sales platform Autotrader, customer review and feedback firm Feefo and dining chain Pasta Evangelists.
The Competition and Markets Authority (CMA) said it was looking into whether Just Eat's ratings system had inflated some restaurant and grocer star ratings, giving a misleading picture of quality.
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For Autotrader and Feefo, the CMA is investigating whether a number of one-star reviews - moderated by Feefo, which handles reviews for the new and used car site - were hidden on the platform and did not count towards the star ratings.
Dignity is under investigation by the CMA into whether it asked staff to write positive reviews about the firm's crematoria services.
Pasta Evangelists, the watchdog said, is being looked at over allegations it offered customers discounts for leaving five-star reviews on delivery apps without this being disclosed.
Since April last year, companies have been banned from certain tactics around online reviews under law, such as fake posts, paid-for reviews that are not clearly marked as incentivised, as well as for hiding negative feedback.
The CMA has already secured commitments from Google and Amazon to beef up their systems to identify and remove fake reviews.
In Amazon's case, it promised tighter scrutiny and sanctions for rogue sellers and businesses falling foul of the rules.
Sarah Cardell, chief executive of the CMA, said: "Fake reviews strike at the heart of consumer trust - with many of us worrying about misleading content when looking at reviews online.
"With household budgets under pressure, people need to know they're getting genuine information - not reviews or star ratings that have been manipulated to push them towards the wrong choice.
"We've given businesses the time to get things right. Now we're deploying our new powers to tackle some of the most harmful practices head on."
The CMA has the power to issue fines as a last resort, if firms found to be breaking the law fail to comply with any demands made by the regulator.
A spokesperson for Just Eat said it was working closely with the CMA to make sure reviews were transparent, clear and easy to use for all customers and that it would continue to "engage constructively" with the investigation.
Feefo said it was fully supportive of the CMA's objective and remained "entirely confident" in its compliance framework.
"Feefo was founded on the principle of transparency. Our platform is engineered to ensure that every review is rooted in genuine consumer intent, backed by a fair, evidence-based process for ensuring the authenticity of feedback for both consumers and dealers," it added.
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Autotrader said that it always aims to operate as a "responsible and compliant business and will cooperate fully with the CMA's investigation".
A spokesperson for Dignity said the firm was taking the CMA's concerns "extremely seriously" and was fully cooperating with its investigation into the Crematorium and Memorial Group - a business division within the company.
"We are committed to ensuring full compliance with consumer law and will continue to engage constructively with the CMA throughout this process," they added.
Pasta Evangelists said it takes the "integrity and transparency of customer reviews extremely seriously" and was committed to ensuring that its practices were fully compliant with consumer law.
"We are cooperating fully with the CMA as it works to understand the facts, and the CMA has itself made clear that no conclusions have been reached," it added.
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