The Government embarked on its flagship Universal Credit scheme without knowing how it would work and has already been forced to write off more than £30 million in failed IT, according to the public spending watchdog.

In a damning report, the National Audit Office (NAO) said the project championed by Work and Pensions Secretary Iain Duncan Smith had been beset by "weak management, ineffective control and poor governance".

Universal Credit is due to replace a bundle of means-tested benefits by 2017, with the department estimating it will save £38 billion in administration, fraud and error costs by 2023.

The system is also designed to encourage people to take up work - by ensuring they will be better off through having a job.

However, pilots have been scaled back and delayed, and a former Olympics executive was drafted in earlier this year to "reset" the programme.

The NAO said the Government had not achieved "value for money" on its spending up to the end of April.

Of the £303m spent on IT, £34m had been written off and the systems still had "limited functionality".

"Throughout the programme the Department has lacked a detailed view of how Universal Credit is meant to work," the report said. "The Department was warned repeatedly about the lack of a detailed 'blueprint', 'architecture' or 'target operating model' for Universal Credit."

The NAO added: "These concerns culminated, in October 2012, in the Cabinet Office rejecting the Department's proposed IT hardware and networks."

The auditors found the IT system could not identify potentially fraudulent claims, meaning manual checks were needed.

A DWP spokesman said the assessment did not cover the progress since April.

"Universal Credit is a major and complex reform that will transform the welfare state and we are committed to delivering it on time by 2017 and within budget," he said.

The spokesman added: "The NAO itself concludes that Universal Credit can go on to achieve considerable benefits for society."