Thousands of retired police officers and firefighters have had to wait up to 15 years to receive their full pension entitlement due to errors in the way it was calculated, the Whitehall spending watchdog has said.
The Government accepted 34,000 people who retired between 2001 and 2006 had been underpaid after the Pensions Ombudsman ruled in May 2015 that the Government Actuaries Department (GAD) had been guilty of maladministration in a test case brought by a retired firefighter.
The National Audit Office (NAO) said some cases had not been resolved until the current financial year - 2016/17 - even though officials first realised there was a potential problem as far back as 2002.
The NAO said the total underpayments were estimated at £711 million - an average of £21,000 for each pensioner affected.
The problem arose over the calculations used by GAD to convert annual pension entitlements for those who chose to commute them into lump sum payments.
"Government's statutory responsibilities in relation to the review of commutation factors were not sufficiently understood in the period leading up the pensioners' complaints," the NAO said.
The chairman of the Commons Public Accounts Committee, Meg Hillier, said the issue should have been resolved sooner.
"Whilst these underpayments in themselves are concerning, what is even more alarming is that these retired personnel have had to wait up to 15 years to get their full payouts when the problems were picked up early on," she said.