The delayed £300m Child Support Agency (CSA) IT system developed by EDS is processing only one in three applications, the Liberal Democrats claim.
But the government says the figures overlook the progress that has been made.
The system was developed to support a new formula to calculate maintenance payments. It was due to go live in April 2002, but has only been used to process new applications since March this year.
According to the Lib Dems fewer than four per cent of the 150,000 applications being run on the new system are actually receiving payments, and nearly one in five assessments are incorrect.
The Department for Work and Pensions (DWP) said: 'The picture these figures paint is misleading.
'We have cleared more than three times the applications in the second quarter of operation than we did in the first,' it said.
Lib Dem work and pensions spokesman Steve Webb said: 'CSA staff are having to get pocket calculators out simply to keep the backlog down.'
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