A quarter of people claiming money for mis-sold payment protection insurance (PPI) did not know companies acting for them would take a fee, research has shown.
In a survey of more than 2,000 people, 25% admitted not knowing claims management companies (CMCs) charge, usually 25% plus VAT.
Only half of people (49%) knew that using a CMC would be no more successful than making the claim themselves.
Just over a quarter said the main reason they would use a CMC, if making a future claim, is that they would not know how to make a claim themselves.
More than half of people (53%) who have used a CMC to get back mis-sold PPI said they would be unlikely to use one again.
The biggest complaint was about value for money, with 37% of consumers unhappy.
The research was undertaken by MoneySavingExpert.com and Which? as they launch an advertising campaign to tell the public how simple it is to claim for themselves.
Also, the groups are set to hold a PPI summit with major banks and credit card providers with the aim of working together to "restore trust" in the reclaiming process.
Which? executive director, Richard Lloyd, said: "This new research shows that many people don't realise there are simple, easy to follow steps to reclaim their money for free and so they're turning to claims management companies, too often unaware they charge a fee."
MoneySavingExpert.com's Martin Lewis said: "Sadly PPI has become a cash bonanza for unscrupulous claims companies, who through a series of lies and misdirection are persuading people they are the only option for getting their PPI money back."