The Child Support Agency faced fresh controversy last night when 85% of absent parents' maintenance payments were found to contain errors.

Sir John Bourn, head of the National Audit Office, refused to accept the CSA's annual accounts because of the high level of mistakes over maintenance payments.

In one in six of the cases, the error was for more than #1000, according to survey by the NAO.

The agency's performance in its early years had left ''a legacy of error'' which continues to affect amounts paid in child maintenance, the survey said.

But, in a glaring contradiction, the agency claimed in its annual report that 87% of cases checked in March this year were accurate to the last penny.

It also claimed a 33% increase in the amount of cash collected from absent parents over the last year, with 98% of cash passed to the parent caring for the child within 10 days.

The agency came under fierce attack in the first couple of years after it was set up in 1993 to track down absent parents, mainly fathers, and extract maintenance payments for their children.

But recently it had appeared to improve its performance.

However, last night Sir John reported that his staff had identified errors in 85% of maintenance balances from absent parents which they examined.

The NAO survey indicated that 39% of receipts from absent parents were for wrong amounts, mainly because of mistakes made by the agency in making assessments.

Sir John estimated that over payments in 1996/97 amounted to #3.8m, some 1.8% of the #215m collected, and underpayments to #9.4m, some 4.4% of the sum collected.

There was also ''a material level of error'' in the amounts the agency claimed were owed by absent parents, amounting to #48m in over statements and #91m in understatements.

But although the agency will be embarrassed by the NAO report, there was little respite for customers as the agency said it did not have the resources to review its full caseload to correct the errors.

It said its top priority was to make sure new maintenance assessments were correct and their second priority was to reach agreements on the payment of arrears.

Although the NAO has carried out previous reports into the operation of the CSA, yesterday's report is the first time it has examined maintenance payments collected by the agency.

Some 74% of assessments made by the agency in 1996/97 were accurate, according to the NAO.

But the survey found that in previous years the level of accuracy was much lower - 63% in 1995/96 and only around 50% in the agency's first two years 1993/94 and 1994/95, creating the legacy of errors

Meanwhile in its annual report published yesterday, the CSA claimed it had collected almost twice as much money from absent parents - #400m - than the NAO figure.