Consumers need to hunt for deals as they do on the High Street when using switching websites for energy, holidays or insurance, a review has found.
Price comparison websites worked best for car insurance and worst for broadband, the study by the Competition and Markets Authority (CMA) said.
Consumers should use a variety of sites to get the best deals, it said.
The CMA said it was investigating one website over contracts that risked raising home insurance prices.
A spokesman for Comparethemarket.com said: "We operate some simple contractual arrangements with home insurers to ensure that our customers always get the lowest available price. We welcome the opportunity to discuss this further with the CMA."
Andrea Coscelli, CMA chief executive, said it was "vital" that people benefitted from using comparison sites given that most have used one.
"The good news is that more than 90% of the people we surveyed were very or fairly satisfied with the sites they used. But we have also found that improvements are needed to help people get even better deals."
Tips for using price comparison websites
- Choose carefully between comparison sites, like you would any retailer
- Not all sites are the same, so try more than one if you can
- Check how the site has ordered results
Price comparison websites offer best buy tables and, increasingly, automated switching services that do not charge a fee to consumers.
Instead, companies often pay a fee of about £30 to a site for each new customer they gain.
The CMA study into what it calls Digital Comparison Tools (DCTs) looked at sites offering price comparisons for car insurance, home insurance, energy, broadband, flights, and credit cards.
Despite a general clean bill of health, the CMA raised concerns including:
- Sites risk breaking equality laws by not doing all they can to make tools user-friendly for vulnerable people
- The need for more explanation on what is done with personal data
- Potential issues with the presentation of excess amounts on insurance deals
- One site's contract that limits insurers' ability to offer cheaper deals on other comparison sites - possibly resulting in higher home insurance prices for consumers
The final point has led to the launch of an investigation against one site. It is against the law for a site to stop businesses offering cheaper deals elsewhere.
The CMA has detailed a template for these sites to ensure they deal with customers fairly. Last year, the CMA's review of the energy sector found that more than a third of the 7,000 people it asked did not realise switching suppliers of gas and electricity was an option.
Alex Neill, from consumer group Which?, said: "When shopping around via comparison websites or apps, consumers should be confident that they can find the best deal for them and that their personal data will be used responsibly. All regulators must ensure businesses abide by the rules and customers get good outcomes, or be ready to take action."
Recent proposals from regulators have included a plan from Ofcom to allow mobile phone users to switch operators by sending a text to the provider they want to leave.
Energy regulator Ofgem said it was considering a trial that would allow households to enter only their postcode and current supplier into a "digital deal checker" to view the cheapest tariff, and a clickable option on all price comparison websites to ensure customers saw prices in the whole of the market, not just preferred suppliers.