The LG Philips display factory in Newport is to close.
A downturn in the market has been blamed for the cuts
Workers on the night shift at the plant have been told that all 870 jobs at the site are to go.
The cuts are being blamed on a downturn in the market and increased competition.
Phil Styles, general manager of manufacturing at Newport, said the decision to close had been made with great regret.
"It is based solely on the continuing adverse business situation," he said.
"It in no way reflects on the performance of the employees at the plant, who have worked hard and demonstrated considerable commitment over these past five years," he added.
The company says it is now entering a 90-day statutory consultation period with the work force and unions.
Production in Newport will end in the middle of August 2003,
The product has lost its appeal and there is no longer any call for it
Amicus Welsh Secretary Roger James
Economic Development Minister Andrew Davies said the cuts were "extremely disappointing".
Mr Davies said assembly government officials had met LG Philips chiefs to discuss the future of the plant.
He added that he had written to the chairman of the company to ask them to reconsider their position.
"We are reasonably optimistic that most of the employees will be able to find employment relatively quickly and we will be working with the support agencies to ensure that this happens."
He added that it was not a case of LG Philips relocating production elsewhere, and said cuts were not also being made at another nearby plant.
"As far as we are aware, the planned closure does not affect the LG Electronics Wales plant, which assembles monitors and computers."
The Newport plant produces colour tubes for monitors and televisions.
Last September, a leaked document received by the union Amicus suggested LG Philips could be reviewing its UK operations, threatening the Newport plant.
Tory AM for south east Wales William Graham said the Labour-led coalition in the assembly could have done more to stop the jobs going.
The LG Philips plant makes tubes for televisions and monitors
"We had very strong rumours this plant was to close," he said.
Newport West MP Paul Flynn said it was a "bitter blow" to the city and sympathised with the workers.
Welsh Secretary of Amicus Roger James said the assembly must now assist the region.
"I'm going to be looking to the Welsh assembly and the economic development minister for help - we need to ease the extent of the job losses."
"The product has lost its appeal and there is no longer any call for it."
LG Philips has also announced 71 jobs would be lost at a plant in Southport.
In the last few years, Wales has seen the loss of nearly 2,000 television component jobs at Sony, Panasonic and Hitachi plants.