There is unlikely to be an extension to the life of Wylfa nuclear power station on Anglesey, a committee of Welsh MPs has been told.
Wylfa, which opened in 1971, is due close in four years' time
The Nuclear Decommissioning Authority (NDA) blamed the costs involved of keeping it open, in evidence to the Welsh Affairs Select Committee.
Wylfa is due to close in 2010.
The island's council had called for Wylfa to continue in operation but has since backed the idea of Anglesey having a second nuclear station.
The existing power station supplies electricity directly to the metal smelting plant Anglesey Aluminium in Holyhead.
Both plants are major employers on the island and a report has claimed the planned closure of the power station in 2010 would lead to both sites shutting, with a combined loss of 1,500 jobs.
Councillors on Anglesey had originally called for a two-year stay of execution for Wylfa and ultimately for a new nuclear power station - Wylfa B - to be built on the island, near Cemaes.
The nuclear plant uses magnox fuel
In January this year, Enterprise Minister Andrew Davies confirmed he had asked the UK Government to look at keeping Wylfa open beyond 2010.
But giving evidence to the Welsh Select Committee on Wednesday, the NDA's regional director Brian Burnett said Wylfa's future was linked to operations at other sites.
He said the fuel used in the ageing magnox reactors uses a special magnesium metal casing but the factory making them had ceased production.
The body's engineering director, Richard White, responding to a question from Ynys Mon MP Albert Owen, said the NDA was in the process of carrying out a feasibility study on the costs of continuing power generation at the plant beyond 2010.
He said: "At the moment I think it would be fair to say that it's looking unlikely that an overall positive business case would be generated.
"When you take account of the costs of a Wylfa, Springfield and Sellafield operation extension and what all that means, against even optimistic views of the electricity pricing, it's not looking positive at the moment."
Mr Owen said: "There are strong economic arguments to keep it open but it's always likely to be difficult and the NDA's report is likely to say that those technical difficulties are going to be a massive challenge for them."
Both Wylfa and the Anglesey Aluminium works were developed in the early 1970s.