The cost of decommissioning ageing nuclear power sites has risen "rapidly" in the past few years by £12bn to £73bn, according to an official report.
Spiralling costs of decommissioning blamed on "unforeseen expenses"
The National Audit Office (NAO) said costs were rising, even for the most imminent work. It said the industry faced "significant challenges".
The Nuclear Decommissioning Authority admitted costs had risen, but predicted they would fall in the longer term.
Greenpeace says the uncertainty makes building more reactors "reckless".
The estimated £73bn cost of decommissioning the UK's old nuclear sites is 18% higher than an estimate given in 2003.
The NAO report said the sum partly reflected "a more complete assessment of the range of work needed to be done".
Sir John Bourn, head of the NAO, said while there was a better idea about "the scale of the task", the "estimates of costs to the taxpayer had continued to rise".
Edward Leigh MP, Conservative chairman of the Public Accounts Committee, said that while the Nuclear Decommissioning Authority (NDA) had "come a long way in planning what needs to be done", it was "worrying" that estimates for future work were still rising.
"Decommissioning relies in part on income from increasingly unreliable plants, and unforeseen expenses continually pop up.
"These factors combine and disrupt plans, slowing down the decommissioning process," he said.
Ayrshire Hunterston A
Cheshire Capenhurst (fuel facility)
Cumbria Calder Hall, Sellafield, Windscale, LLW Repository
Gloucestershire Berkeley, Oldbury
Kent Dungeness A
Oxon Harwell, Culham JET
Somerset Hinkley Point A
Suffolk Sizewell A
Environmental campaigners said the government should heed the warnings of the NAO report.
Nathan Argent, nuclear campaigner for Greenpeace, said: "That we can't properly estimate the cost of dealing with our legacy wastes, let alone resolve what to do with them, underscores the fact that plans to build a second generation of new reactors are pure recklessness."
In a statement the NDA said that in less that three years since its creation the authority had gained a "detailed understanding" of the sites.
It said it had managed to deliver £300m more work than budgeted, whilst also driving "fundamental change" within the industry.
"Costs of the clean-up programme were always expected to rise in the early years ... [and] we remain confident that ... we will stabilise and then ultimately reduce the UK's nuclear liability," it said.