The number of customers with unresolved complaints against Wonga over mis-sold loans is more than four times the initial estimate given by the Financial Ombudsman Service.
In a letter to the Treasury Committee, Grant Thornton chief executive Dave Dunckley said this means that over 40,000 people could be affected, compared to 10,500 estimated in January 2019.
This came after the Financial Conduct Authority (FCA) confirmed payday lender Wonga Group collapsed into administration in August 2018.
In the letter, Dunckley said that the total number of redress claims is expected to rise when administrators publicly request claims from borrowers who believe they may have been sold an unaffordable loan.
An online portal to help customers make claims will be set up, Dunckley said. He added that using a claims management company would not accelerate the process but could simply end up costing claimants additional charges.
