The report compared five like-for-like car insurance quotes on ten comparison websites and found that it rarely got the same results.

In one scenario of insuring a 22-year-old driver living in a high-risk area, there was a difference of more than £1,500 when looking at the cheapest quotes on each site. 

GoCompare.com, TescoCompare.com and MoneySupermarket.com had the cheapest price at £751.25, with MoneyExpert.com being the most expensive, quoting £2,280.70.

Even for one low-risk scenario, the cheapest quotes varied by more than £150 between sites.

Which? said price variations mean it is more important than ever for consumers to shop around.

The research found MoneySupermarket.com led on coverage with 120 insurers out of an estimated 230 brands in the market, whereas LoveMoney.com and MoneyExpert.com covered 10% of the market by number of brands.

Customer satisfaction for price comparison websites also came low.

Only a third of Which? members said they trusted price comparison sites to find the best price, regardless of supplier.

MoneyExpert.com and MoneyFacts.com were ranked the highest but their customer satisfaction scores were just 46%. 

Bottom of the customer satisfaction score was MoneyExtra.com with a score of 28%, receiving only two star ratings for all categories.

Market leader MoneySupermarket.com, which had an annual turnover in 2012 of £204.8m, was ranked fourth in the table.

The survey found those who use comparison sites visit an average of two sites before making a decision and one in eight feel the need to visit four or more sites to get the best price.

Richard Lloyd, Which? executive director, said: “Price comparison sites claim to do all of the work for you but we found they don’t always guarantee people the best deal, or even the right one.

“We want comparison sites to treat customers fairly by being upfront that they don’t cover the whole market and more transparent about what is and isn’t included in the policies they’re selling.”

The consumer group said the cheapest price is not the only thing to consider, as the investigation found that in instances where an individual didn’t want to pay a voluntary excess had an automatically excess preselected for them.

A spokesperson for uSwitch.com, which scored 39%, said: “We take our role in helping consumers to find the best deal for their needs extremely seriously. We always welcome feedback about our service and we will be taking a look at this report to see what we can do to make it even easier for consumers to save money.”

Advice to people when using price comparison sites includes:

• Check more than one site, as we found you’re likely to get a range of results. Remember that not all sites cover all insurers, and some companies won’t be included as they choose not to be featured on comparison sites.

• Look out for preselected options and added extras, as the sites make assumptions and you may end up with quotes that don’t reflect what you want.

• Consider value for money as well as price, as the cheapest deal may not provide you with the cover that you need.

• You don’t have to buy through a price comparison site and could use the quotes provided to help negotiate a better deal with your current provider.