The life of the only working nuclear power station in Wales could be extended by another nine months.
The ageing Magnox reactor at Wylfa is earmarked for closure in 2010
The Wylfa nuclear station on Anglesey is set to stop generating in March 2010.
The Nuclear Decommissioning Agency (NDA) said it was reviewing the timescale, as it diverts cash resources to other sites.
The decision is also expected to mean decommissioning of the old Trawsfynydd station in Gwynedd will slow down.
The NDA said its priority was to reduce the hazard at sites that need the most work, and the means investing at Sellafield in Cumbria and Dounreay in Scotland.
"For the first time we are now able to take a UK wide, long term look at what needs to be done and when," said NDA chief executive, Dr Ian Roxburgh.
"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."
Under the agency's draft business plan, the Wylfa nuclear power station would still close in 2010, but its lifespan would be extended from March until the end of the year.
The ageing Magnox nuclear reactor supplies electricity to the biggest employer on the island, Anglesey Aluminium, and fears have been raised that when Wylfa shuts the aluminium operations and 1500 jobs will be in jeopardy.
Decommissioning at Trawsfynydd is due to finish in 2096
At Trawsfynydd, decommissioning has been underway since the plant stopped generating power in 1991. That process is set to continue until the end of this century.
But under the new proposals, site operators Magnox Electric are being asked to look at options to reduce spending.
The NDA said realistically that would mean slowing down decommissioning work from spring 2009.
"There was always going to be a period when the decommissioning process slowed down, as a site enters a safe storage period and the area is left for 20 to 25 years," said a spokesman for the NDA.
"We envisage that at Trawsfynydd we may be able to bring that safe store period a little bit forward."
But the NDA insisted that the timetable to return the Trawsfynydd site back to green fields remained in place, with all decommissioning work expected to finish in 2096.