Price comparison website Compare the Market is under investigation over suspicions that it's helped to push up insurance prices.
The Competition and Markets Authority (CMA) is concerned about the way the website has special deals with certain home insurance providers banning them from making cheaper offers elsewhere.
As a result, it says, consumers buying from other companies could end up paying higher prices.
"We have set out ground rules for how sites should behave, as well as being clear on how regulators can ensure people have a better experience online," says CMA chief executive Andrea Coscelli.
"We are also taking enforcement action where we suspect the law may have been broken."
It has the ability to fine offending companies up to 10% of their turnover.
On the whole, says the CMA, the vast majority of people who use price comparison sites are happy with the service they receive. But, it says, there's room for improvement.
It wants sites to be clearer about how they make their money, how many deals they're displaying and how they are ordering the results. Companies often pay to appear in lists, or to be pushed up the rankings, and the site may get a commission when you switch providers.
It's also important to remember that not all companies appear on all comparison sites; some aren't listed on any.
"Try more than one site, or another one next time - they're all different," says the CMA. "Check how many companies they cover. Some sites will show more than others."
Its research shows nearly two-thirds of people using a price comparison site do visit more than one when shopping around.
"If you want more info, look for sites that show reviews and product features. Watch out for adverts mixed in with the results," continues the advice. "Don't forget to visit the sites of businesses not included on the comparison website."
The CMA says it now plans to turn its attention to how well consumers are protected when it comes to car hire companies and hotels.