Anglesey Aluminium smelter at Holyhead will cease production in September, with around 300 people set to lose their jobs.
The company, owned by Rio Tinto Plc and Kaiser Aluminium, Inc, could not reach a sustainable plan for the plant after it had turned down a very generous £48 million package over 4 years from the UK Government.
With the electricity supply contract from nearby Wylfa Power Station coming to an end in September, Anglesey Aluminium Metal had been trying to secure an alternative power supply.
One option they had considered was an extension of the current contract with the Nuclear Decommissioning Authority (NDA), owners of the Wylfa site.
Local MP Albert Owen, who has worked continuously to save the plant along with Welsh Secretary Peter Hain, was deeply disappointed by the decision and said it was a "devastating blow to the island economy".
The smelter had already announced 250 compulsory job losses in July as well as 140 people taking voluntary redundancies. Now there will only be 80 staff retained to carry out a much smaller remelting operation.
Attention will now turn to what plans the company will have for the proposed biomass plant for which it was intending to seek planning permission and carry out an environmental impact assessment.
The Welsh Assembly Government will be working flat out to help people adjust to this shock and find alternative careers in what is already a challenging economic environment.