The CMEC says the latest payment figures are among the best ever
The body that enforces child maintenance is processing more and more cases by hand because its computer systems are flawed, MPs have said.
The Work and Pensions Select Committee said the additional clerical costs of this method were "mounting alarmingly".
But it also said the Child Maintenance and Enforcement Commission (CMEC) had made progress in other areas, with recent pay figures among the best ever.
The body took over from the Child Support Agency (CSA) in 2008.
The committee said the CMEC had made substantial progress in clearing the backlog of cases and speeding up claims - but as of September last year, 27% of absent parents were still not paying maintenance.
And despite a £107m upgrade of computer systems in 2008, the number of cases being dealt with manually had risen from about 19,000 in 2006 to 60,000 by March last year, 75,000 in September and a predicted 108,000 in September this year, the report added.
The CMEC is promising new computer systems next year and says the latest payment figures are among the best ever, with more than 800,000 children benefiting for the first time.
A spokeswoman added: "The [information technology] IT problems of the current CSA schemes are well documented. The new child maintenance scheme, which will be supported by a brand new IT system, will provide the long-term solution to this issue. This remains on track for launch next year.
"The commission is confident that the issue of clerical cases will not impact on its ability to transition to the new scheme."