Decommissioning the nuclear site at Sellafield faces continued delays and an overspend of up to £913 million, according to an official report.
The National Audit Office (NAO) said the Nuclear Decommissioning Authority (NDA) had improved its performance in delivering major projects at the site in Cumbria.
But work is still predicted to be late and to cost more than originally expected, said the spending watchdog.
The NDA's nine major projects were expected to cost an additional 60% of their budget at the design stage in 2015, but this has been reduced to 29%, said the NAO.
While this was a substantial improvement, it was still a forecast overspend of £913 million.
The NAO reported that three projects were cancelled when £586 million had already been spent on them after the NDA said it had found a better way of delivering the work.
It said Sellafield Limited has achieved £470 million in efficiency savings, but added that neither the NDA nor the company knows their make-up and admit that a proportion does not represent genuine efficiency savings.
"The strategic decisions the NDA takes around prioritising activity at Sellafield could be profoundly changed and improved by a better, more evidence-based assessment of these constraints.
"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 report said.
"The Department for Business, Energy & Industrial Strategy's governance of the NDA is complex and not working as well as it should to support improvements at Sellafield,.
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 £120 billion 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."
A Business Department spokesman said: "The NAO review makes clear the improving performance of Sellafield Ltd and the significant progress and value for money it is delivering in ensuring the safety of the Sellafield site. The Government will carefully scrutinise the NAO report and consider its recommendations."
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