More cash will be needed to scrap the Sellafield nuclear reprocessing complex in Cumbria, the Nuclear Decommissioning Authority (NDA) has revealed.
Sellafield has suffered operational difficulties
The authority says ongoing "operational difficulties" mean it will take longer than expected to make safe old-style Magnox facilities at the site.
The NDA is to spend £8.5bn over the next three years - an increase of £671m on the previous three-year period.
Most of the cash will go on work at Sellafield and Dounreay in Scotland.
The figures are revealed in the NDA's draft three-year plan.
Chief executive Dr Ian Roxburgh said: "In line with our strategy, our priority is hazard reduction and we will be focused on the sites that require the most work.
"This means that the majority of funds over the next three years will be focused on Sellafield and Dounreay, whilst safety remains the absolute priority across all our sites.
"It is increasingly clear that due to operational difficulties at Sellafield, the timescales for defuelling the Magnox stations will need to be re-assessed and we will need to work through the implications of this with our stakeholders."
Barry Snelson, managing director at Sellafield, said: "We have received a higher level of funding than in previous years, and this will allow us to maintain our focus on reducing the high-level hazards on our sites.
"While this will mean a reduction in the rate of growth at Sellafield, it clearly affirms our strategy of putting cleanup of the highest hazards as our first priority.
"We will need to increase the timescales associated with our clean-up plan but, on the plus side this should help deliver a longer, more stable work schedule rather than peaks and troughs."