Kerry Chilvers, head of marketing at Direct Line said: "We are delighted that Which? has shone a bright light on the insurance aggregator market. Providers in this space are not always what they appear.
"Aggregators can imply they cover the whole of market, when many only cover a select range of companies that pay to be included. Products such as Direct Line's are not quoted or sold via these channels.
"We would support moves to enforce greater transparency in the sector with these websites to be encouraged to inform consumers if they are owned by a major insurer, when they are promoting 'exclusive' deals.
"Whilst comparison sites provide a useful overview of what's on offer, the on-line aggregator approach is not necessarily a short cut to value, service or quality. They do not always compare like for like products and consumers do not always realise they are commercial businesses.
James Daley, editor of Which? Money, said: 'Some of the websites do not highlight the small print and you are encouraged to make a decision based on price, which might be great when you pay but not great when it comes to making a claim.
'They are not always working in the best interests of the consumer. In the worst cases they are misleading, which is why greater scrutiny of the industry is merited.
Daley said many sites inflate the amount of voluntary excess in car insurance quotes to push down the price.
In response, Confused's head of insurance, Gareth Kloet, said: "The questions that a customer will answer from site to site will not be identical, and offers and exclusives can also vary between comparison sites."
Moneysupermarket's insurance expert, Peter Harrison, admitted the website had exclusive deals with some companies. "That way we can get customers great deals," he added.
A spokesman for Comparethemarket said: "Every customer presents a different set of requirements and so all quotes provided are tailored specifically to their needs.'
John Miles, business development director at GoCompare, added: "There are a lot of different permutations and we try to be as accurate as possible."