Parents were overcharged by around £7.6 million in child maintenance last year because of faulty Government assessments of their cases, a public spending watchdog said as it declined to fully sign off the scheme's accounts.
Official errors also accounted for £6.1 million in underpayments, the National Audit Office estimated.
It said the scale of official error - which has contributed to "significant and unresolved inaccuracies" in calculating a multibillion-pound arrears total - meant it was unable to give approval.
Responsibility for the long-troubled scheme was transferred from the Child Maintenance and Enforcement Commission to the Department of Work and Pensions in 2012.
It has upgraded IT systems to produce more "robust" calculations of arrears, which is presently estimated to have reached £3.85 billion since the scheme began in 1993.
But the NAO found that still "does not give a true and fair view" of the situation because of the cumulative impact of incorrect assessments over the years which are not rectified.
Setting out his decision, the head of the NAO Amyas Morse said assessment errors were primarily down to "caseworker error and system problems rather than complexity of legislation".
"However, owing to the scale and age of the accumulated issues, there remain significant and unresolved inaccuracies in the reported arrears balance."