Comparison sites are failing to provide consumers with the best deal and should be strictly regulated, Which? has said.
The Financial Services Authority warned comparison sites they must hold the correct permissions for their activities in June last year.
A Which? investigation claimed that many of the 11 most popular sites were using pre-selected answers and assumptions in their online forms for insurance products.
"This may make quotes appear cheaper, but can also lead to basic ‘one size fits all' policies that could be unsuitable for individual needs or even prove invalid in the event of a claim," the report said.
Which? executive director Richard Lloyd said: "We want to see tougher regulation and monitoring of price comparison sites to make sure they treat customers fairly and transparently."
This call came after the British Insurance Brokers Association (BIBA) demanded targeted regulation for the sites in August 2011.
In May 2011, research from SaveitBuddy.com claimed that hidden comparison site commissions hit £650m.
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