The hot strip mill at Llanwern steelworks in south Wales is be temporarily mothballed, with the loss of 115 jobs.
Tata Steel blamed the move on "continuing poor UK steel demand".
The GMB union described the decision as "a devastating blow for the UK steel industry".
Welsh Secretary Cheryl Gillan said it was important that businesses in Wales remained flexible in changing economic circumstances.
Tata employs about 7,500 staff in Wales, with about 1,400 at Llanwern in Newport.
Last month the company announced it was cutting 70 jobs by closing its construction products business at the same site.Keith Hazlewood, GMB national secretary, said the announcement "yet again demonstrates what a roller coaster industry the steel industry is".
He added: "This decision is a devastating blow for the UK steel industry, the economy and the local community of Newport, south Wales and the supply chain in the surrounding area, especially at this time of the year."
Mr Hazlewood said the GMB along with the other steel unions would work with the company "to mitigate any compulsory job losses."
Mrs Gillan said she understood the disappointment being felt in Newport as a result of Tata's decision.
But she added: "I am aware this is not the first time the company has taken this decision and I remain optimistic that the rolling mill will be brought back on line when economic circumstances allow.
"Given the difficult economic climate faced by all businesses in Wales it is important both they and government - whether in London or Cardiff - remain sufficiently flexible to adapt quickly to changing circumstances."
Mrs Gillan said that while the UK government was putting in place measures to support business, the Welsh government also had a key role to play in helping the local economy and create jobs.
"Today's news underlines the continued importance of our two governments working together, rather than in isolation, in the interests of the Welsh economy," she said.