The National Audit Office has been unable to give a full sign off to the accounts of the Child Maintenance and Enforcement Commission for 2008-09 and 2009-10.
The head of the NAO, Amyas Morse, rejected the accounts after discovering millions of pounds of wrongly made payments and has questioned the accuracy the CMEC's figure of £3.69bn for the backlog in uncollected debts.
The NAO estimates the irregular receipts and payments for 2008-09 are £13.5 million overpayments and £15.5 million underpayments, and for 2009-10 are £10 million overpayments and £14.4 million of underpayments.
Mr Morse has also given adverse opinions on the reality of the outstanding child maintenance arrears, which the Commission reports to be £3.717 billion at 31 March 2009 and £3.694 billion at 31 March 2010. The NAO say that these figures do not give a true and fair view because of the level of error in the underlying case data.
However the head of the NAO said that since the Commission took over responsibility for the statutory child maintenance schemes, it has made significant improvements to the information available on child maintenance arrears, meaning that errors in the underlying case data, which were previously unknown, have now been made visible.
"Since the statutory child maintenance schemes were introduced, there have been problems with the accurate calculation of maintenance and with the two underlying IT systems, neither of which was capable of properly reporting arrears," Mr Morse said.
He added: "The Commission inherited these problems from the Child Support Agency. Accuracy of maintenance assessments continues to be a challenge. The Commission is continuing to improve the accounting information available, so that the historic problems affecting the accuracy of arrears data are more visible. However, the Commission still has a significant challenge in collecting the arrears, which have accumulated since the beginning of the maintenance schemes."