Sir John Bourn, the head of the National Audit Office (NAO), refused to pass the agency's accounts for the fifth year running because of inaccuracies.
One-third of absent parents were paying the wrong amount, he said. Four out of five also had their payments recorded inaccurately once they had been made.
Out of pounds 409.8m received from absent parents, Sir John said pounds 23.8m was overpaid. Underpayments amounted to pounds 6.2m.
David Davis, the chairman of the Commons' committee which oversees NAO reports, called for urgent action to tackle the legacy of error in the CSA.
Although some progress had been made in the last year, the proportion of absent parents paying the wrong amount had actually risen, he said. "These errors are not notional accounting issues, they represent injustice.
"For every person who gains from an error there is a loser. Unless this is addressed as a matter of urgency the agency will be beset by problems for a generation."
Last year the agency was criticised for aiming at just 85 per cent accuracy, but this year it set an even lower target of 75 per cent.
Mr Davis also pointed to a 250 per cent increase in the number of compensation payments made as "a clear measure of the poor performance of the agency". The rise did show, however, that the agency is getting to grips with its backlog of cases.
Sir John's report said parents' individual maintenance accounts, which contained pounds 511m on 31 March this year, had overstatement errors amounting to pounds 79.4m.
Others contained errors which led to an understatement of the amount which had been paid by a total of pounds 53.4m.
There were fewer errors in interim accounts set up before a full assessment had been made.
Three out of five of them contained errors, and in a total balance of pounds 178.7m there were overstatements of pounds 28.6m and understatements of pounds 1.2m.
The agency has long suffered from extensive problems, and last year its staff turnover was 27 per cent.
This month the government published a White Paper containing plans for reform of the agency.
In the CSA's annual report, published yesterday with Sir John's assessment alongside it, the agency's chief executive Faith Boardman admitted: "The first six years of the CSA have not been easy. The agency had a difficult and controversial start and faced criticism from the public and Parliament."
But, she went on, most people had now come to accept that it was the duty of parents to support their children financially.
"The CSA starts from the moral high ground," she wrote in the report's foreword. "But I'd be the first to acknowledge that our administration of the child support system has not been good."
Ms Boardman added that enormous changes had been made in the last year and more were to come.
More maintenance has been collected from non-custodial parents, mostly absent fathers, and more paid to those looking after the children, usually the mothers.
Improvements had also been made to the CSA's telephone service, she said.Reuse content