A government watchdog has criticised the body overseeing the Sellafield clean-up.

It has recommended a review into the role of the Nuclear Decommissioning Authority, because it is "unclear" what it does.

The report praised it for improving its performance in delivering major projects, as most delivered their work to schedule and to budget in 2017-18.

But it said the NDA could not demonstrate how its current work led to progress against the long-term decommissioning "mission".

The report also identified an expected £913 million overspend and delays in clean-up projects at the West Cumbrian nuclear complex.

It added: "The NAO has found that the role of the NDA is unclear and this could put at risk the progress we are now seeing at Sellafield.

"The Department for Business, Energy and Industrial Strategy’s governance of the NDA is complex and not working as well as it should to support improvements at Sellafield."

It added that despite the authority and Sellafield reporting £470 million of efficiency savings, they could not explain the make-up of those savings.

It said: "They admit that a proportion do not represent genuine efficiency savings."

The NAO has recommended that the role, function and governance of the NDA are reviewed, and that the NDA improves its understanding and communication of progress at Sellafield, as well as the constraints to faster and further progress.

The report, published today, says since its last report in 2015, significant progress had been made with programmes to reduce risk and hazard in legacy ponds and silos.

The NDA also expects it will reach critical milestones with legacy facilities.

The report also highlights that most major projects at Sellafield delivered its work to schedule and to budget in 2017-18.

However, major projects are still predicted to deliver late and to cost more than the NDA originally expected.

In 2015, the NDA’s nine major projects which were in construction were anticipated to cost an additional 60 per cent of their budget at design stage.

This has been reduced to 29 per cent over budget – a forecasted overspend of £913 million.

The report said that evaluating overall performance at Sellafield was difficult due to a range of factors.

The NDA has cancelled three projects after spending £586 million on them, saying it found a better way of delivering the work.

Legacy ponds and silos programmes have also delivered less work than originally planned in three out of the past six years, but are still expected to reach critical milestones early.

The NAO said: "Evaluating overall performance is also complicated by the fact that the NDA has not yet been able to demonstrate how its current work leads to progress against the long-term mission.

"The NDA states that factors other than funding, such as physical constraints of the site, limit how fast they can achieve progress in the reduction of high hazard at Sellafield, yet it has not tested these factors sufficiently."

The National Audit Office added that the NDA's strategic decisions could be changed and improved by a "more evidence-based assessment" of the constraints.

It said the NDA's unclear role was underlined by the recent failure of the contract to decommission its Magnox sites.

It said: "It provides an important imperative for a review of how the NDA performs its function, and how the government department supports it in doing so."

David Peattie, NDA chief executive, said: “We welcome the report, which reflects the hard work going in to building a more effective NDA, our focus with Sellafield on making and demonstrating better progress in the clean-up and decommissioning work, and the positive effect of making Sellafield Ltd a subsidiary of the NDA.

“However, as the NAO rightly highlights, there is still much more to be done, including on how we better evaluate and demonstrate progress.

"The recommendations of this report will inform and support our ongoing work to improve and strengthen the role of the NDA and continue our upwards trajectory of progress at Sellafield and across our other 16 nuclear sites.

“I would like to thank the NDA and Sellafield workforces for their continued commitment to this vitally important work.”

Amyas Morse, head of the National Audit Office, said: "The improvements in reducing risk at Sellafield are encouraging, but the scale of the challenge is very great and the department could be doing more to support the NDA through better governance and oversight of performance.

“The NDA, for its part, needs to do a better job of explaining what progress it has made and what it will achieve over the next two to four years so Parliament can hold it to account.

"It might also help if there was less focus on the extraordinarily round terms of £120bn to be spent over 120 years, and more focus on what can be delivered in a more meaningful timescale, say within 40 years, in terms of cleaning up this extremely dangerous nuclear legacy.

"I would hope this could be a great deal if they push ahead.”

The NDA officially took over the running of the site on April 1 2016 and Sellafield Ltd – the firm responsible for the clean-up – became a subsidiary of the NDA.

The change was initiated in January 2015 when the Government stripped private sector consortium, Nuclear Management Partners (NMP), of its contract to run the site, ending its controversial six-year tenure.