Ministers say the CSA is making progress.

The Department for Work and Pensions said that at the end of March there were 195,000 uncleared applications - a drop of 31% over the previous year.

But the Conservatives said that some of the most complicated cases were not included in those figures.

They also said the CSA had missed official targets on accuracy and debt.

Absent parents

Figures, published on the Department for Work and Pensions (DWP)'s website, said the CSA's caseload stood at 1.4 million at the end of March 2007, a decrease of 2% at the same point last year.

Of all new applications received in December 2006, 61% were cleared within 12 weeks, compared with 53% in December 2005.

Of the applications received in September 2006, 78% were cleared within six months - up from 67% in September 2005.

Lord McKenzie, a minister in the Department for Work and Pensions, said: "The CSA has made encouraging progress, improving its performance and getting more money to more children than ever before, with 635,000 children now benefiting from maintenance payments and an additional �50m being collected from absent parents compared with this time last year.

"This is a result of the huge effort and determination of the agency's employees."

'Worrying performance'

He added: "But we know that many parents still have significant problems with the system.

"This is why we are replacing the CSA with a new child maintenance system."

But the Tories said that 28,000 of the most complicated cases were not being systematically included in the CSA's figures - a 50% increase on last year.

Shadow minister for family welfare Maria Miller said: "Hiving off the most difficult cases is distorting the true picture and could be masking even more worrying performance problems in the CSA.

"The government is letting down thousands of families as there is no way of knowing how quickly or accurately these cases are being dealt with."

The Tories also said the CSA had missed targets set by the DWP in terms of maintenance outcome, accuracy and debt.

A spokeswoman for the CSA said some of the 28,000 cases referred to by the Tories included cases where maintenance was being paid.

She added that some were complicated cases while others had been removed from the computer system because they had caused "glitches".

The government previously announced plans for a new organisation, the Child Maintenance and Enforcement Commission, to replace the CSA.