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 said the Nuclear Decommissioning Authority 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.
The NAO reported that three projects were cancelled when £586million 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.