Such websites often imitate those of genuine health insurance brokers to capture leads, which are then sold on to authorised firms.
eSmart Media Ltd, which runs the website bestmedicalcover.co.uk, was censured in October 2013 for scaremongering by the Advertising Standards Agency after it claimed that the NHS had been responsible for 13,000 needless deaths since 2005.
Bestmedicalcover.co.uk was ruled as a 'health insurance comparison website' by ASA.
Brian Walters, AMII vice-chair, said: “We have found over 25 of these websites and there are doubtless others.
“Although the small print usually makes clear that leads will be passed to an authorised intermediary, use of the first person such as 'we work with all the top UK private medical insurers' misleads consumers into believing that the website is operated by a broker.”
The AMII letter states that companies who operate such websites are unfairly competing with intermediary members, while they could damage the reputation of the health insurance industry.
It is added: “Firms that shoulder the significant burden of regulation deserve a properly policed market.
“These websites are competing for business with authorised brokers and they are able to be much more aggressive in their marketing because they are under no obligation to ensure that their promotions are clear, fair and not misleading."