A consumer watchdog has called for tougher regulation of price comparison sites to ensure they treat customers fairly and transparently.
The Which? consumer group tested 11 of the most popular sites and found many of them using pre-selected answers and assumptions in their online forms.
While this could make quotes appear cheaper, it could also lead to basic "one size fits all" policies that were unsuitable for individual needs or even invalid in the event of a claim, the watchdog said.
A Which? researcher applied for car insurance quotes from 11 popular comparison sites in February, looking at the five cheapest quotes from each site. Despite entering the same details every time, prices varied from £310.28 to £660.20.
A Which? survey of 9,197 members in January to coincide with the study found overall customer satisfaction with comparison sites has dropped from 46% to 42% in the last year.
It also found that even the highest-rated comparison sites were on a par with the lowest-rated insurance companies.
The study found that none of the 11 sites covered more than a third of the market for home or travel insurance despite their suggestion that customers had no need to look anywhere else for the best deal.
Which? executive director Richard Lloyd said: "Price comparison sites claim to do all of the work for you, but our research found if you want to get the best deal you still need to shop around or it could cost you hundreds of pounds. Going to the insurer directly may also give you a better quote.
"We want to see tougher regulation and monitoring of price comparison sites to make sure they treat customers fairly and transparently."
The watchdog advised customers to check all questions for pre-selected answers, get quotes directly from companies and beware of unwanted extras.