Child maintenance arrears are understated by the Government by tens of millions of pounds in some cases and overstated by millions in others, the Whitehall spending watchdog said.
The National Audit Office (NAO) gave an "adverse" - the most serious - opinion on the "truth and fairness" of the Department for Work and Pensions (DWP) calculations of arrears on the 1993 and 2003 child maintenance schemes.
The department calculated a total of £3.976 billion in arrears at the end of the last financial year.
But the NAO said the figure does not give a "true and fair view" because of the level of error in the underlying data - "a result of both inaccurate maintenance assessments by caseworkers and incorrect processing of cases".
"The best available estimates of the impact of incorrect processing of cases indicate that the reported arrears at March 31 2016 are overstated by around £15.9 million and understated by around £115.8 million," the watchdog said.
The 1993 and 2003 schemes are closed to new cases but they still run concurrently with the new 2012 scheme.
Under the old schemes, child maintenance overpayments amounted to £3.3 million while underpayments totalled £4.5 million.
A DWP spokesman said: "We are committed to giving every child the best start in life and are focused on ensuring today's children get the right level of support, so we actively pursue parents who aren't meeting their financial responsibilities.
"The reality is, in nine out of 10 cases, parents are paying towards their due child maintenance.
"We know the old system wasn't good enough, which is why we replaced it in 2012 with a much more accurate system."