Price comparison websites are misleading people who buy insurance on the internet, a report gave warning today.

Defaqto, a research company, accused the sites of falsely claiming to offer car insurance quotes from the whole market and offering inaccurate quotes.

The report, which investigated leading sites including Moneysupermarket.com, Confused.com, GoCompare, Comparethemarket and TescoCompare, found that major insurers are often not listed and that no insurer appears on all of the top ten sites. Defaqto said that the so-called "aggregator" sites make incorrect assumptions about a customer and manipulate levels of cover to make the policies look cheaper, for instance, by automatically giving a customer a higher excess than they might want.

The inconsistencies mean that car owners must still go through three or four comparison sites to really compare the market, defeating the purpose of the sites, Defaqto said.

The research group also criticised the sites for encouraging drivers to select policies based on price alone.

Mike Powell, general insurance consultant at Defaqto, said: "People using these sites should be careful to check exactly what they are being sold. In particular, they should pay close attention to exces levels and policy add-ons. Customers should also think carefully before buying the cheapest policy as often the cheaper policies will not offer extra benefits like a courtesy car or assistance following an accident."

The Resolution Foundation recently called for the Financial Services Authority to regulate the price comparison market like banks and building societies because of concerns over the influence they now have over consumer's decisions.

James Harrison, chief executive of Insurancewide.com, a price comparison site, said: "Good aggregators do provide a really useful service, but you need to do your research to see through marketing messages and to be sure you're visiting a true aggregator, not just a broker."

The Defaqto report gave TescoCompare the highest rating, above more long-established sites such as Moneysupermarket.com, even though the site compares quotes from fewer insurers.