The estimated cost of cleaning up the Sellafield nuclear reprocessing site in Cumbria has risen by almost £2.5bn in a year, a report has said.
The Public Accounts Committee (PAC) said clean-up costs at the complex had risen from £67.5bn in 2013 to an "astonishing" £70bn.
The report said progress had been "poor" and targets had been missed.
The consortium in charge of the clean-up said the challenges at Sellafield had been "unprecedented".
The report by the committee of MPs made a series of recommendations, calling on the Nuclear Decommissioning Authority (NDA) to terminate the contract of the private consortium Nuclear Management Partners (NMP) if its performance did not improve.
The report said the consortium had been brought in six years ago to help Sellafield improve its performance and had its contract extended last October despite "spiralling costs and poor performance".
Margaret Hodge, who chairs the committee, said: "Cleaning up the nuclear waste on this hazardous site is estimated to cost more than £70bn in cash terms. What's worse is that the cost is likely to continue to rise.
"The Nuclear Decommissioning Authority, which owns Sellafield and which appointed NMP, said itself that it did not expect NMP to meet its savings target for the first five years - despite NMP being on course to earn £230m for the job.
"Timescales have slipped and reprocessing targets have been missed. NMP has failed to provide the clear leadership, strong management and improved capabilities for the job."
Ms Hodge said the authority should monitor progress and terminate the contract if NMP's performance does not improve quickly.
'Number of disappointments'
NDA chief executive John Clarke said: "We have made good progress towards implementing the recommendations laid down by the PAC and will now focus on achieving the aims of the recommendations set out in this latest report.
"Both NMP and the NDA now have a much better understanding of the issues and complexities that exist at the site and the challenges that lie ahead.
"Whilst progress has been made on a number of fronts, we will require significant improvements during the next contract period.
"We have had extensive discussions with NMP and made clear where these improvements must be made."
NMP chairman Tom Zarges said the challenges at Sellafield were "unprecedented, with complexities exceeding any other operational or decommissioning nuclear site in the world", which needed "extraordinary technology and skills".
"The first term of our contract has been characterised by many successes, but also a number of disappointments and areas for improvement.
"Our job now is to build on our experience of the last five years to safely and reliably deliver our customer's mission, while further accelerating the pace of change and providing value for money to the NDA, government and the UK tax payer."
A Department for Energy and Climate Change spokesman said the findings of the PAC would be taken "very seriously" and their recommendations would be considered.