The trading watchdog has written to 100 major price comparison websites amid concerns a lack of confidence and trust is preventing some people getting the most out of them.

The Office of Fair Trading found some comparison sites could be clearer about the way search results are presented and make it more obvious which businesses people are dealing with.

It said more should be done by some sites to step up privacy policies and complaints procedures. Its review said a lack of trust means some people are missing out on savings, although the emergence of the websites has been a major step forward.

The OFT's market study into internet shopping in 2007 estimated consumers stood to gain extra savings of £150 million to £240m a year using such websites.

But Clive Maxwell, OFT chief executive, said: "Not all sites have the same standards and we are working to ensure consumers are empowered to make informed choices. We hope this will improve trust among consumers who do not use price comparison websites, and who may be missing out on savings."

The OFT said if people do not want their information to be shared with other companies, they should check what "opt out" options are on the website.

Comparison sites often take information about those using them, which can be used for marketing.

Consumer Affairs Minister Jo Swinson said: "When a consumer uses these sites they believe they are being savvy. It's important their trust is well-founded."