Aviva has set aside £323m to deal with underpayments on pensions, insurance and savings products affecting more than four million people.
The Daily Mail reports the insurer has known about hundreds of technical errors since 2007 and has been setting aside money to compensate individuals, with compensation running to thousands of pounds in some cases.
The insurer underpaid people with personal pension plans, workplace pensions, life insurance cover and savings, many of whom are still to be contacted about the mistake.
It is understood that Aviva has written to four million policyholders, beneficiaries and trustees to inform them of the issues.
The newspaper says Aviva has paid out £180m to affected customers with a further £143m still to come.
Problems are said to have stemmed from the merger of Norwich Union and CGU in 2000.
Following complaints to the Financial Ombudsman Service over policy inconsistencies, an investigation was launched which found wrong income tax rates had been used on pension plans, there were issues with how pensions had been invested, and some people had been told the value of their funds would not lose money.
In a statement, Aviva told the newspaper: “At Aviva we believe in doing the right thing for our customers. We regret that these historical mistakes were made. If any error has been made we will put it right.
“Since 2007 we have strengthened our processes and controls to prevent these types of issues happening again. We are working through any outstanding cases systematically and are making good progress. Any customers who may be affected can be confident we will contact them.
“We found the issues, we are fixing them and we are putting them right.”