Claims management companies have pocketed up to £5 billion of consumers' compensation for mis-sold payment protection insurance, according to estimates from Citizens Advice.
The charity, which analysed some previous figures from the Financial Ombudsman Service (FOS) to make its projections, also carried out consumer research suggesting that nearly one third (28%) of people who have used a claims management firm felt under pressure to pursue a claim.
Nearly two-fifths (39%) of people who had used a claim firm said they were not aware they could have made the claim themselves, according to a survey of more than 5,000 Britons.
Meanwhile, almost half (47%) of people who used a claim firm said if they had known about the free help available they would not have used the firm.
Figures released by the ombudsman service showed that it took on a record 575,836 new complaints from consumers about financial firms in 2013, up by over one third (38%) on the previous year.
Of the cases referred to the ombudsman in the second half of last year, three quarters (76%) were about PPI.
Template letters that people can use to complain are available on various websites, including the FOS website and consumer help website MoneySavingExpert.com.
Citizens Advice chief executive Gillian Guy said: "Consumers have lost out on billions of pounds worth of compensation because banks were too slow to get a grip on the PPI scandal.
"By banks originally dragging their feet and providing inadequate redress, claims management companies seized an opportunity to take up to 25% of people's compensation for admin work that consumers can do themselves for free.
"Some claims management companies operate well below the standards that are expected and sometimes outside of the rules.
"The regulator needs to quickly revoke the licences of firms that are not up to scratch. We'd like to see a ban on cold calling by claims companies which would spare people from nuisance calls and protect consumers from predatory firms."
The Government has been clamping down on rogue claims management firms.
Around 200 claims firms have had their licences removed over the last year according to figures recently released by the Claims Management Regulation Unit at the Ministry of Justice.
Action against claims management companies that are not up to scratch has been stepped up over the last year, as part of wider action to stamp out bad practice, including bombarding people with misleading advertisements and cold calling.
Changes to "no-win no-fee" claims were introduced last spring as part of a package of measures to help cut insurance costs.
These include a ban on referral fees paid between lawyers and claims firms, while claims management companies are also not allowed to take fees from customers before a written contract has been signed.
The total number of such firms has plummeted by more than 1,000 since a peak of 3,367 in 2011 to 2,254 by November last year.