An "intolerable risk" is being posed by hazardous waste stored in run-down buildings at Sellafield nuclear plant, a watchdog has found.
The National Audit Office (NAO) also said that for 50 years, the operators of the Cumbria installation failed to develop a long-term plan for waste.
Costs of plant-decommissioning has also spiralled out of control, it said.
Operator Sellafield Ltd, said it welcomed the report's findings and was "making improvements".
The plant is the UK's largest and most hazardous nuclear site, storing enough high and intermediate level radioactive waste to fill 27 Olympic-sized swimming pools.
The NAO report states however, that owners of the station do not know how long it will take to build storage and treatment centres for the hazardous material or how much the final bill for decommissioning the plant is likely to be.
'Ripe for dithering'
It also concluded that over the five decades it was open, operators failed to plan how to dispose of the radioactive waste and some of the older facilities have "deteriorated so much that their contents pose significant risks to people and the environment".
A long-term plan to clean up the Nuclear Decommissioning Authority-owned site, was agreed last year after an earlier one stalled because it was "unrealistic".
Margaret Hodge, who chairs the public accounts committee, said: "Projects of this length and ambition are ripe for dithering and delay.
"I am dismayed to discover the clean-up of Sellafield is no different. The authority's revised plan sees critical milestones shunted back by up to seven years.
"Hazardous radioactive waste is housed in buildings which pose 'intolerable risks to people and the environment'.
"My concern is that unless the authority holds Sellafield Ltd to a clear and rigorously benchmarked plan, timetables will continue to slip and costs spiral."
The NAO report concluded that progress in 12 of the 14 major buildings and equipment projects considered "critical" for reducing risk, which range in cost from £21m to £1.3bn, also failed to achieve what they were supposed to and had not provided good value for money.
Dr Ruth Balogh, of West Cumbria and North Lakes Friends of the Earth, said: "The UK's failure to deal with highly hazardous nuclear waste at Sellafield is a national scandal that poses a significant risk to local people and the environment.
"The government has completely ignored the urgent need for interim measures to deal with this radioactive waste.
"We shouldn't build any new nuclear reactors if we can't deal with the radioactive mess that's already been created."
Around 240 of Sellafield's 1,400 buildings are nuclear facilities and so far 55 buildings on the site have been decommissioned.
Amyas Morse, head of the National Audit Office, added: "Owing to historic neglect, the authority faces a considerable challenge in taking forward decommissioning at Sellafield.
"It is good that the authority now has a more robust lifetime plan in place but it cannot say with certainty how long it will take to deal with hazardous radioactive waste or how much it will cost."