Indian-owned Tata Steel has confirmed that it will close 12 plants in Britain.
The move will result in 900 job losses, the company confirmed, including 580 in South Wales, 155 in Yorkshire, 120 in the West Midlands and 30 on Teesside.
Sites to close include Tafarnaubach and Cross Keys in South Wales, and it will also reduce shifts at Rotherham and Hartlepool in response to lower demand for products.
The chief executive of the company's European operations, Karl Kohler, said the move was part of a strategy to become an "all-weather steel producer", able to withstand the difficult economic conditions.
Mr Kohler added: "The job losses are regrettable and I know this will be a difficult and unsettling time for the employees and their families affected.
"We will be working with our trade unions and government at a national and local level to ensure we provide them with as much assistance and support as possible."
The company employs 19,000 in the UK and said it remained committed to investing in the business to help create long-term stability.
It confirmed plans to re-start one of two blast furnaces at Port Talbot in the first quarter of next year as part of a £250m investment programme.
Michael Leahy, general secretary of the Community trade union, said it was "sad news" for those affected by the job losses.
"We will be seeking an urgent meeting with the company to ensure our principle of no compulsory redundancies is upheld, although we are pleased to see the company has already committed to offering a package of training and support for those affected by these changes," he said.
"Sadly, these potential job losses are symptomatic of the continuing failure of the Government's economic policy and yet another reason why we are calling on the British Government to take urgent action to stimulate economic growth ana help revive the manufacturing sector."
A Welsh Government spokesman added: "This is very disappointing news, and a massive blow to those who will be losing their jobs.
"Tata's decision reflects the serious and ongoing challenges faced by manufacturing industries during these very difficult economic times.
"In addition to these challenges, it is clear that high energy costs and uncertainty over UK Government energy policy are having a significant impact on business investment decisions.
"As a Government, we have warned for some time of the need for these costs to be reduced."