More than 90 jobs are expected to go at the Corus tinplate works in Llanelli, Carmarthenshire.
Trostre manufactures tinplate used to make food and drink cans
The company is expecting a reduction of 94 jobs as it cuts its output at Trostre as part of a review because of "difficult market conditions."
Corus said the latest job losses would help secure the long-term future of the plant which employs 800 people.
But union leaders reacted angrily, saying Corus had failed to consult with them before making the announcement.
Corus said from January 2008 it would reduce its overall manufacturing capacity at Trostre to match expected customer demand.
The plant produces steel used in the manufacture of food and drink cans and other products.
Earlier this year about 650 of the 800 workers were told to take two weeks off on half-pay after a slowdown in orders.
Works manager Stuart Wilkie said: "We have an excellent record of improvement at Trostre, however we are experiencing very difficult market conditions.
"To ensure the longer-term security of packaging steels manufacture at Trostre we must take action to reduce costs and improve our operational flexibility."
He said full consultation with employees and trade union representatives would begin immediately and a support programme for those made redundant would be provided.
But Michael Leahy, general secretary of the steelworkers' union Community, said he would be seeking an urgent meeting.
"We have not been consulted by the company and we are incensed about today's announcement," he said.
"Corus have not complied with the information and consultation agreement that we have with them and we will be writing to the chief executive asking for an urgent meeting to discuss this matter.
"We have authorised our members at local level to take any action that they deem appropriate given the breach of the agreement."