More than 230,000 retirees have been underpaid the state pension by £9,000 on average, experts say.
Around 237,000 people are affected, and as many as 700,000 people are due reviews to check if they are too, according to financial firm Standard Life.
Affected pensioners can include widows and widowers, divorcees and more. Retirees can be underpaid the state pension for a number of reasons.
