The Eaton Electric plant will move work to mainland Europe
An electronics factory is facing possible closure with the loss of 265 jobs on Anglesey.
Eaton Electric, which makes miniature circuit breakers in Holyhead, would close the plant in December 2009.
The company said it reflected efforts to "increase efficiency and continue to optimize global competitiveness".
It plans to move work to plants in Europe where production costs are up to 35% lower. The deputy first minister said it was a "very grave blow".
Workers were told they could go home early on Thursday after being told of the consultation over possible closure.
I want the workers at Holyhead to know that the assembly government is ready to do everything within its powers to influence Eaton Corporation to maintain a presence on the Holyhead site
Deputy First Minister Ieuan Wyn Jones
The Birmingham-based company said the first job losses, if they go ahead, will start next March.
A spokesman said in a statement: "We greatly respect the outstanding employees at this facility and appreciate their hard work and dedication in this highly competitive environment."
The company said it would offer "extensive support" to workers in finding new jobs and would offer a severance package.
"Eaton also appreciates the tremendous support and hospitality that has been provided to the company and its employees by the local community and regrets the impact this may have," said the spokesman.
The changes follow the US-owned company buying the Moeller electronics group and wanting to transfer production there.
The Holyhead factory makes three million circuit breakers a year but Moeller in Europe makes 60m a year, giving it significant economies of scale.
The workers were allowed home on Thursday
You may as well close the town down
Holyhead councillor Cliff Everett
Deputy First Minister Ieuan Wyn Jones will meet the corporation's operations director for Europe on Friday.
Mr Jones, who is also the local assembly member, said: "I know that the company has always been more than satisfied with the skills and commitment of its Anglesey workforce and its relationship with the assembly government..
"The threat to the future of the operation stems from Eaton's purchase of other electrical companies which make 20 times more circuit breakers a year than the Holyhead plant. The company wants to standardise on the continental product.
"However, I want the workers at Holyhead to know that the assembly government is ready to do everything within its powers to influence Eaton Corporation to maintain a presence on the Holyhead site where it has operated so successfully."
Holyhead councillor Cliff Everett said he knew a number of husband and wife teams at the factory.
"It's not a question of losing one job in a family but losing two."
He said the area was already under the "black cloud" of job losses at Anglesey Aluminium.
"You may as well close the town down. If there's no money being spent in the town, then the knock-on effect could affect hundreds of jobs."
The Holyhead plant closure would take place from March to December 2009.
It opened as the Midland Electric Manufacturing Company in 1960, and was taken over by Eaton five years ago.
Meanwhile, 54 people are losing their jobs at a company in Newport, south Wales manufacturing components for electrical equipment.
International Rectifier Limited said the jobs are being cut at its Cardiff Road plant to save costs.
Some workers have already left and the remainder will go by the new year.