The Foreign and Commonwealth Office (FCO) has written off more than £450,000 in losses after overpaying some members of staff.
Between 2008 and 2015 the department overpaid 699 employees £577,000.
In the financial year ending March 31 2016 the FCO wrote off £135,279 because the cost of recovering very old and very small payments would outweigh the payments themselves.
After trying to recoup the remaining balance, a further unrecoverable £321,858 was written off, bringing the total to £457,137.
The FCO's latest annual report states: "The FCO made an Administrative Loss of £321,858.45 by overpaying staff overtime between April 2008 and March 2015.
"An error in the FCO's payroll system led to an overpayment in overtime claims.
"This affected 699 staff in one pay band. The error has been corrected and, following discussions with HM Treasury, recovery action was initiated.
"The write off represents the balance the FCO has been unable to recover from staff."
An FCO spokesman said its audit department had investigated the cause of the error and that new controls have been put in place to fix the issue.
He added: "An error in the FCO's payroll system led to an overpayment in the rate paid on some overtime claims dating back to 2008 for a small percentage of staff.
"It is unlikely these staff were aware of the overpayment."