The government is repeating the mistakes it made with technology at the Child Support Agency, according to an official audit, which revealed a near doubling in cost for its new IT system.

The National Audit Office said that the Child Maintenance and Enforcement Commission, which has replaced the CSA, had a plan to cut costs by charging single parents for using the service and introducing a more efficient IT system, costing £275 million and supplied by Tata Consultancy Services.

Forecast costs for the new IT system went up by 85 percent last year, it said – the original budget for the system had been £149 million. Over a quarter of this was estimated by the commission to be due to pure increased cost for the same work, and the rest to added functionality - but the NAO said it was unable to confirm this. The Child Maintenance and Enforcement Commission is facing a £44 million shortfall in its £161 million cost cutting target.

Ongoing technical problems have resulted in over 103,000 cases having to be managed manually, the National Audit Office said, at a cost of £48 million.

Supplier management had also been an issue until last year, when contract managers were appointed to liaise with Tata – previously this had fallen on the programme team.

Many of the mistakes were repeated from the CSA, the National Audit Office said in its report. "The original plans were optimistic and the commission lacked sufficient internal resources to understand fully how the IT system would be developed.

"From mid-2011, the Commission reverted from an 'agile' (iterative) development approach, to a more traditional approach to developing systems and strengthened governance arrangements. It is not yet clear whether such remedial actions have sufficiently addressed the earlier problems."

The commission had also wasted money until recently by having duplicate IT, HR and finance functions, the National Audit Office found, as a legacy of the old CSA.

The previous system at the CSA was developed by supplier EDS (now HP), and was heavily criticised after it was found to have over 1,000 problems.

"Faced with a challenging but achievable target for reducing its spending, the Child Maintenance and Enforcement Commission is relying heavily on the introduction of fees to parents, underpinned by a new IT system," said Amyas Morse, head of the National Audit Office. "This is a high risk approach with no contingencies if it goes awry."

TCS referred comment to the Department for Work and Pensions, which said it "welcomes" the report. The DWP added that the new setup would be a "more cost-effective solution with more families putting in place the financial support children need".