Comparison sites, used by millions to choose car and home insurance, were accused last week of misleading consumers.

Sites such as moneysupermarket.com and confused.com do not give a true picture of the “excess” — the amount of a claim you must pay yourself, according to FWD Marketing, a research company.

In a separate report, the Financial Services Authority (FSA), the City regulator, also accused sites of “misleading” practices that could leave consumers worse off.

Comparison firms are increasingly influential in the insurance industry. Almost 25% of all private motor insurance sold in 2007 was through a comparison site, accounting for 30m quotes.

Insurance firms usually have a voluntary and a compulsory excess. The compulsory excess cannot be changed and can vary from £100 to more than £400. Consumers can also opt for an additional voluntary excess to reduce their premiums further.

Some comparison sites, such as Moneysupermarket, list only the voluntary excess. It means you may be offered a cheap quote with a voluntary excess of £100, but when you go through to an insurer, the total excess — voluntary and compulsory — could be much higher.

In some cases, the level of excess shown on a comparison site is less than a third of that quoted by the insurer. So a buyer who thought he would have to pay £100 towards a potential claim would in fact pay £300.

The FSA research investigated 17 sites, which cover 95% of the comparison-site market. It found some sites provide a figure called the “total excess”, when in fact it is the voluntary excess. In one case, a further compulsory excess of £320 was applicable.

Moneysupermarket lists only the “voluntary excess” while beatthatquote.com and confused.com, list “approximate excesses” and gocompare.com lists a “total excess”. Uswitch, however, details the voluntary and the compulsory excess separately.

FWD Marketing, which works with the British Insurance Brokers’ Association (Biba), found some sites such as Confused also increased the level of excess automatically so that cheaper policies came top of their searches. For example, FWD requested policies with an excess of £100, but the top three results had excesses of £575 (Tesco), £300 (A&L) and £200 (ChoiceQuote).

Another £100 had to be added to the A&L cover, as Confused included only voluntary cover.

Confused said it guaranteed quotes would be the same or cheaper when you got through to the insurer though the total excess might be higher. It said: “With the volume of quotes we go through it is difficult to monitor every single one that is sent through to us from insurers. However, if it consistently happens, we do take action to stop it.”