Some 2.5 million customers are to have their payment protection insurance (PPI) complaints re-opened to make sure they have been treated fairly.
Banks, credit card providers and personal loan companies that sold PPI have agreed to the re-assessment of claims that were potentially unfairly rejected or resulted in inadequate compensation between 2012 and 2013 after being asked to do so by the Financial Conduct Authority (FCA).
Announcing the review - which is already underway - the regulator stated companies are getting better at dealing with PPI complaints.
More than 13 million complaints have been dealt with since 2007, with 70% of claims being upheld in the consumer's favour. Some £16 billion has been paid out in compensation since January 2011, which was when the FCA began monitoring payments.
The proportion of complaint decisions overturned by the Financial Ombudsman Service has also fallen from 88% in 2011 to 60% in 2012 and 56% in 2013.
However, the Ombudsman has had more than one million complaints from people dissatisfied with the outcome of their complaints, a figure that equates around 25% of all rejected complaints.
Martin Wheatley, chief executive officer, at the FCA, said: "Making sure anybody previously mis-sold PPI is treated fairly now, and paid redress where it's due, is an important step in rebuilding trust in financial institutions. In around two and a half million complaints this was not necessarily the case so, at our request, firms will be looking at these complaints again."