The Child Support Agency's enforcement unit is costing more to run than it recovers from parents, it has emerged.
The unit aims to recover unpaid maintenance from absent parents
The unit costs an estimated £12m a year to run, but managed to recover only £8m from absent parents last year.
The Department for Work and Pensions said the £8m figure was only "the tip of the iceberg".
It represented what was collected through initial contact with parents and did not include what they would pay in future, a spokeswoman said.
The figures emerged after a parliamentary question from shadow work and pensions secretary Paul Goodman.
Mr Goodman said the CSA was "in a mess" and demanded an urgent statement on its future from ministers.
The CSA recovers maintenance from absent parents for the upkeep of their children.
It can retrieve unpaid child maintenance from absent parents in a number of ways - by taking court action against debtors, using bailiffs, and on occasion confiscating driving licences.
But it has come under repeated fire since it was set up in 1993, with Tony Blair admitting recently it was "not suited to the job".
Three weeks ago, the prime minister told MPs it was "extremely difficult" to make the CSA - an investigating, adjudicating and enforcement agency all in one - cost-effective.
He was responding to claims that £1 was spent by the agency on bureaucracy for every £1.85 that got through to children.
Suggestions the government was planning to scrap the agency were played down by Downing Street.
BBC political correspondent Norman Smith says officials believe improving the work of the enforcement unit is central to the agency's future.
A review of the agency is being carried out by its chief executive Stephen Geraghty and is due to report at the end of this year or early in 2006.