The plant is trying to reach a new deal for cheap power
Two hundred and fifty jobs are to go at Anglesey Aluminium which has failed to reach a new power deal for cheap electricity.
Staff are being informed of the fresh cuts at the Holyhead plant which are in addition to 140 voluntary redundancies announced last week.
A joint statement from the company and the Unite union said primary aluminium production there will end in September.
Earlier this month the firm rejected an offer of £48m in UK government aid.
It said the amount was not enough for the company, Wales' biggest electricity consumer, to break even.
Anglesey Aluminium David Bloor managing director said they have been in negotiations with the UK government for three years.
250 job losses at Aluminium plant
"We've been working with them to try to get power sorted out," he said.
"In the absence of no breakthrough we've had to make some plans so it's not a done deal.
"That said we always wanted to do that in the first place, it's work we've had to do in the absence of no power.
"It's an extremely generous offer and we're grateful for that but as a business we'd lose substantially more than that if we were to keep in business beyond September this year.
"It's not money we pocket as a profit it would cost us an awful lot more to keep in business and that's why we've had to say 'Thanks, but no thanks'.
Mr Bloor said it was important to recognise what was causing the problem and that was electricity prices.
"People talk about subsidies and rescue packages. I think you've got to get to the heart of what's going on here and understand why a business such as ours cant be competitive anymore," he said.
In a joint statement on Thursday, the firm and the union said "despite considerable efforts" no solution has been "found yet regarding a commercially viable power contract for the long term future" of the plant.
Around 470 people are employed at the plant, but with the voluntary redundancies announced earlier this month and the latest 250 job cuts, that number will be reduced to 80.
The statement said that Anglesey Aluminium has now begun preparations for the orderly closure of the smelting activities at Holyhead, while at the same time working on detailing proposals to try to preserve other parts of the business.
The firm is also working with the assembly government and Anglesey council to identify and foster job creation projects.
Anglesey Aluminium said it still intends to continue with plans to build a biomass power station next to its factory.
The deputy leader of Anglesey council, councillor Bob Parry said that the decision was "very sad" but they had expected further job cuts.
Mr Parry told BBC Radio Cymru that Anglesey Aluminium have been offered a very lucrative deal by the UK government but that the decision to refuse it suggested that the company had decided to get rid of the jobs beforehand.
He said more should have been offered to the company, and he suggested that, had the same situation arisen in south Wales, a better deal would have been put on the table.
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