A report has suggested that the closure of Wylfa nuclear power station in 2010 could mean the loss of 1,500 jobs and £42m to the economy of Anglesey.
The nuclear plant, opened in 1971, is due to close in four years
The study, seen by BBC Wales, was commissioned by the island's council and the Welsh Assembly Government.
It concluded that Wylfa's shutdown would also mean the likely closure of Anglesey Aluminium, in Holyhead.
Deputy council leader Gareth Winston Roberts has said the repercussions present a "mega problem" for Ynys Mon.
The power station and the aluminium smelting works are two of the biggest employers on Anglesey, providing nearly one in 10 jobs.
Wylfa supplies electricity directly to Anglesey Aluminium, which saves an estimated £4m a year on the cost of its transmission.
The report, commissioned by Anglesey County Council, the Welsh Assembly Government, Wylfa and Anglesey Aluminium, said that when Wylfa shuts down, Anglesey Aluminium will almost certainly close as well.
It has described the likely impact of Wylfa's closure on the area's economy as "profoundly adverse" and "potentially long-lasting".
Mr Roberts said: "That's a mega problem for Ynys Mon. These two are the major employers here. Our GDP (gross domestic product) is the lowest in the UK."
The local authority is calling 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.
Wylfa and Anglesey Aluminium are major employers in the area
Mr Roberts said: "We have to fight for an extension for Wylfa power station. A, we need that extension for a breathing space because of lack of commitment or a clear guidance from the government and b, we should be backing Wylfa B."
The Nuclear Decommissioning Authority (NDA) said it understood concerns about the economy, had contributed to studies and was "determined to address these issues.
The NDA said: "However, there is no realistic possibility of an extension to the plant¿s life beyond 2010 for a number of technical reasons. One of these is that the only plant which makes fuel for Wylfa is closing down in March this year.¿
Andy Shirt, licensee of the Seventy Nine pub in Holyhead, said: "If Anglesey Aliminium closed it would have a huge impact on the local economy.
"Not only would it have a massive impact on those losing their jobs, but on people employed in ancillary jobs such as those working in a bar-restaurant like we have here."
Neil Crumpton, of Friends of the Earth Cymru, said the organisation was against the idea of extending the life of the existing power station.
He told BBC Wales: "It's a safety case, and it's an increasingly dangerous case because the graphite cores are deteriorating year on year.
"We would like to see far more jobs in renewable industries, in possibly a gas power plant at Anglesey Aluminium to solve the supply concern and provide jobs in other industries.
"The base case is 400 jobs will be lost at Wylfa anyway whether they manage to extend it a couple of years or more, while other things can be done to support Anglesey Aluminium."
Last month Economic Development Minister Andrew Davies confirmed he had asked the UK Government to look at keeping Wylfa open beyond 2010.
First Minister Rhodri Morgan has also told AMs the administration was still against any new nuclear stations for Wales.
The last nuclear power plant to be built in Wales, the Magnox plant at Wylfa, opened in 1971 and produces around 10% of Wales' energy needs.