The LG Electronics plant in Newport, south Wales is to close at the end of the year with the loss of 315 jobs.
The closure has been blamed on the falling cost of LCD monitors
LG Electronics Wales, which assembles computer monitors on the site, blamed falling prices and expressed regret.
It is the only surviving LG factory on the site following a landmark investment in 1996, promising 6,000 jobs. Most failed to materialise.
One worker said staff were very depressed after being told the news before being sent home for the day.
The man, who did not want to be named, said staff were then sent home for the day.
He said the news "even though it was not unexpected, was gutting".
In a letter to workers, the company's managing director said there had been a "great deal of speculation" about the plant's long term future.
Production lines had been reduced from six to three but a return to profit "unfortunately...has not materialised."
The letter added: "The decreasing sales price of LCD computer monitors together with the increased competition within the market makes it impossible to operate this site profitably and this downward trend cannot be reversed."
"It is a huge disappointment to us all and the decision was made with great regret."
The LG site has had a troubled history, with up to 6,000 jobs promised in 1996 never materialising.
A £1.2 billion sister factory on the site, which was to make semi-conductors, never went into production and is still empty.
- The LG Philips factory making colour tubes for monitors and televisions closed in 2003 with 870 job losses.
- At its height, 2,000 people were employed at the site.
- The LG factories on the site originally received more than £87m of grant money, and in 2005 some £34m was repaid to the then Welsh Development Agency.
Irish company the Quinn Group, making radiators, took over the LG Phillips plant in 2005, creating 460 jobs.
Brian Morgan, of Cardiff Business School, was chief economist for inward investment body the Welsh Development Agency when the factory was set up.
He said the project was "doomed from the start" adding that the overall plan was "far too ambitious".
He said: "It was a massive attempt to bring an electronics factory and semiconductor plant to Wales and it never got off the ground."
Enterprise Minister Andrew Davies said he was "very disappointed" with the closure decision.
"The company has assured us that this decision in no way reflects on the quality of the workforce in Newport, but has been taken as a result of the intense global conditions for their products - particularly the rapid decline in price of the products they supply," he said.
Mr Davies said a team would be sent in to work with the company and the employees affected by the decision.
"We are also examining the local supply chain to ensure that any impact of this planned closure on any local companies is minimised," he said.
Liberal Democrat Kirsty Williams said: "Instead of the major shot in the arm that the Welsh economy was promised, Newport got a pain in the neck - a small factory producing electronics and an embarrassingly large hi-spec white elephant."