Millions of consumers use price-comparison websites to search for the best deals on everything from car insurance to gas and electricity, but their impartiality has come under question after it emerged that some plan to charge firms to be included in their best-buy tables.

The websites, which are supposed to scan the internet to find the best financial or energy deals, have soared in popularity over recent years. More than 15m people used a comparison website last month, thinking it would help to save them money, according to Comscore, an internet analyst.

However, Confused.com, one of Britain’s largest comparison websites specialising in insurance, has come under fire after it said it would only include in its tables companies that pay a monthly fee. The firm, part of insurance group Admiral, said that from next month it will charge any insurance company wishing to be included in its comparison tables between £7,500 and £15,000 a month. Consumer groups are concerned that many firms will refuse to pay the fee, skewing the information provided, meaning that consumers miss out on the top deals.

James Davey of Norwich Union said it was considering whether to pay the fee. He said: “We’re the country’s biggest insurer but we’ve not yet decided if we want to accept this charge. Smaller insurers are likely to be even less keen.”

Until now, comparison sites have not charged firms to be included in their tables. Instead they have made money by charging for additional services such as advertising or for signing up customers via their website.

Laurence Trim of insurer Swiftcover said: “Comparison sites are so powerful because they have as wide a selection of firms on their sites as possible. If some remove themselves from Confused because of the higher fee it will seriously undermine its credibility.”

Debra Williams at Confused dismissed this. She said: “A couple of firms have indicated they wanted to discuss the new fee. Some will accept this and some won’t, but I expect most of our partners will come round to it eventually.”

Moneyfacts, one of the most highly regarded comparison sites, has also said it will introduce a charge for firms that want to appear in its best-buy tables.

Moneyfacts isn’t demanding the fee itself, but in future it will only list firms that subscribe to Datascreen, which is used by banks, building societies and independent financial advisers to monitor the rates on different products.

One of the firms that is unhappy is Kent Reliance building society, whose cash Isa is a regular feature in the best-buy tables. Chief executive Mike Lazenby said it had been quoted £11,000 to subscribe to Datascreen. He said: “Moneyfacts has told us categorically that we will not be on their best-buy tables if we do not pay the fee. This means the data they put in their best-buy tables isn’t necessarily the truth.”

However, Andrew Hagger at Moneyfacts said the move would benefit users. He said: “Datascreen helps us produce a more accurate picture of what’s available. We would not be happy including a small organisation we had barely heard of that simply told us it was offering a high rate. I don’t think that would be in the best interests of the consumer.”

Even sites that do not charge a fee for inclusion are not always as comprehensive as they seem because most don’t scan the whole market. Insuresupermarket, part of the Moneysupermarket franchise that provides The Sunday Times’s best-buy tables, covers 50 car insurers while Confused includes 42 companies in its search. Uswitch, which only recently began covering insurance, searches for the best deal among 25 providers.

Some firms refuse to be listed on comparison tables. The Royal Bank of Scotland group, which owns Direct Line, Churchill and Tesco Personal Finance, does not appear on any comparison sites and it has recently begun an advertising campaign highlighting this.

Direct Line said: “We firmly believe that people can get better deals on insurance by cutting out middlemen and dealing direct with insurers.”

Switching sites can also overstate potential savings because most produce quotes based on the bare minimum of information needed. Incorrect assumptions mean that the final quote can be hundreds of pounds more than the comparison site states.

A search using Moneysupermarket for home and contents insurance for a professional living in London last week produced a best-buy premium from Marks & Spencer of £397.26. Going through to the M&S website, however, further questions were asked that produced a final annual premium of £633.83 – more than £236 dearer than the original quote.