Tata Steel will mothball parts of its plant in South Wales leading to some 250 people to be made redundant.
The job losses in the Llanwern plant are expected to affect mostly agency workers, sources familiar with the situation said.
"This is more bad news for the steel industry,” said Dave Hulse, national officer of the GMB union. “This comes on the back of the recent announcement that 720 jobs will go in the Speciality Business.”
The union said it has genuine concerns over the future of the steel industry in the UK and wants to meet with Prime Minister David Cameron to discuss the situation.
"We will see local communities destroyed with mass unemployment while the balance of payment deficit will increase further if we don't act now and get Government support and action,” Hulse said.
The union is concerned about energy prices which have a negative effect on the industry's competitiveness and wants the government to secure better tariffs for large energy consumers.
"We fully acknowledge that Tata Steel, and indeed the entire UK steel industry, is under huge pressure to reduce costs and become sustainable,” said Roy Rickhuss, general secretary of Community and chairman of the National Trade Union Steel Co-ordinating Committee. “Cripplingly high energy costs combined with the strength of the pound and a flood of Chinese imports are killing the industry and we desperately need Government to stand up for steel and make a game-changing intervention.”