Water firm Severn Trent has confessed it has given industry regulator Ofwat more false information.
The company says it had misreported statistics about the way it handles complaints from customers and enquiries about bills.
The admission echoes a similar problem revealed last month at Southern Water, which will lead to a fine for Southern.
Severn Trent has already been punished for reporting false data to Ofwat about its income and bad debts.
The latest problems, over customer service data, were first discovered in February.
“We have been reporting more favourable data than we should have” said a company spokesman.
Severn Trent said it did not know how much money was at stake but admitted the problems seemed to stretch back several years.
“We are determined to resolve any issues uncovered as soon as possible,” said chief executive Colin Matthews.
“We shall continue to work in full and open co-operation with Ofwat and obviously if customers have been disadvantaged then we shall reimburse them.”
A final report will be ready in June when Ofwat will launch its own investigation using independent accountants.