The Ministry of Defence has apologised after thousands of recently-retired veterans were underpaid their pensions.
More than 5,000 ex-military personnel who have left the armed forces since April last year are receiving about £200-£300 a year less than they should, the BBC reported.
The problem arose because the Service Personnel and Veterans Agency failed to update the way it calculates entitlements when veterans choose to "commute" part of their pension so they receive a bigger lump sum on retirement.
The mistakes were uncovered by Commander Michael Goldthorpe, who left the Royal Navy last year after 33 years' service.
He said he was "disappointed" that many of those affected have still not been told that they are being underpaid, telling the BBC: "The MoD were happy to sit back and not inform them they were being paid incorrectly."
The Service Personnel and Veterans Agency said the problem will not be fully rectified until their computer systems are upgraded in September or October, the BBC reported.
An MoD spokesman said: "We are aware of an inaccuracy with a relatively small number of armed forces pensions 'commutation' calculations since April 2010 and we are upgrading our IT systems to address this.
"Those affected will be fully reimbursed and we apologise for any inconvenience caused."