They warn that families who rely on these websites could miss out on hundreds of pounds on the best credit card and loans deals.

Price comparison websites have increased in popularity during the recession as consumers use them to hunt down the best possible deals, with over 22 million people using one of these sites in the last three month.

However, many consumers do not realise that comparison sites make their money by earning a commission every time a consumer “clicks through” to a financial service provider’s website to apply for a bank account, loan or insurance product.

On average the commission equates to about £40 for an insurance deal, with it rising to £150 on the most profitable loans. Some price comparison sites only list a selection of deals, excluding those companies that will not pay a commission.

The majority of sites attempt to list all providers, regardless of whether they pay a commission, but do not display the commission-free deals on their home page, or they make it quite difficult to find the commission-free deals.

Because most of the sites refuse to list deals that will not pay a fee on their home page it is very difficult for consumers to see the best possible deals.

For instance, one of the highest rates available on an easy access savings account with a £10,000 minimum deposit is The Ulster Bank’s Pathway account, paying 3.6 per cent for the first six months.

However, it is not listed on the first page of Moneysupermarket.com’s easy access accounts or uSwitch’s initial search results for instant access accounts.

The deal is on both of these comparison sites, but consumers need to unclick the “accounts available on uSwitch” before it appears and users need to exit the “best sellers page” on moneysupermarket and conduct a full search. Even after doing that, users need to scroll down past the “sponsored products” to the “full results” to see it.

The Financial Services Authority has given comparison websites a clean bill of health, but the Office of Fair Trading earlier this year started an investigation to ascertain whether consumers were being misled.

It is understood that the OFT has concerns that the sites too often skew certain aspects of a deal – such as high excess amounts on insurance deals – in order to present some products in the most appealing light.