More than a million complaints have been submitted to car finance firms in the past month using a tool created by Martin Lewis’s Money Saving Expert website.
The tool, which helps users send complaints about mis-selling to companies without using a claims firm, has been used at a rate of 30,000 a day since its launch at the start of February.
MoneySavingExpert.com has said this could equate to £480m being returned to consumers.
The finance regulator, the Financial Conduct Authority (FCA), launched a major investigation into hidden, unfair car finance commission in January.
Before January 2021, some lenders allowed brokers to adjust the interest rates they offered customers for car finance, known as a discretionary commission arrangement.
These created an incentive for brokers to increase how much people were charged for their car loan, the FCA said.
It banned the practice in 2021, but has said there have since been a high number of complaints from customers about how much they were charged before the ban.
It said providers were rejecting most of these complaints because they believed they had not acted unfairly or caused customers to lose out, but that it was assessing the extent of the problem.
Mr Lewis has encouraged people to claim now, arguing that there could be a risk that a time limit will be placed on complaints.
He said those who could be affected include people who bought a car, van, campervan or motorbike on either a personal contract or hire purchase deal for primarily personal use between April 2007 and 28 January 2021.
Mr Lewis added: “The numbers of complaints in not much more than a month is staggering – off the charts, far more than I expected.
“So it’s not surprising that some firms are struggling to respond to complaints in a decent time.
“To frustrated complainers, I’d say that, for now, we should be prepared to give companies some wriggle room on timings, but firms need to urgently step up their complaint handling resources. And this is just the beginning.
“Even though we were at the vanguard of PPI and bank charge reclaiming, in terms of numbers of complaints, this feels like it is building up even more quickly.”
The tool can be found here.