Steel giant Tata is cutting 900 jobs and closing 12 sites under plans to improve competitiveness amid a slump in demand across Europe.
Most of the job losses will be in South Wales, including 500 at the Port Talbot plant, although sites in Yorkshire, Teesside and the West Midlands will also be hit.
Unions described it as "devastating" news, especially so close to Christmas, while Unite said it rounded off a week of "jobs carnage" across the UK economy.
Sites to close include Tafarnaubach and Cross Keys in South Wales, while shift levels at the company's Rotherham and Hartlepool plants will be reduced to match production to lower demand for bar products and pipelines.
Tata said demand for steel in Europe had fallen by 25% since 2007 and was forecast to slump by another 10% this year, with construction among the industries cutting back.
The Indian-owned company, which employs 19,000 in its steel business in the UK, said it remained committed to investing in the business to help create long-term stability. It also announced that it will re-start one of two blast furnaces at Port Talbot in the first quarter of next year, later than planned, as part of a £250 million investment programme.
Karl Kohler, chief executive of Tata Steel's European operations, said: "These proposals are part of a strategy to transform ourselves into an all-weather steel producer, capable of succeeding in difficult economic conditions.
"These restructuring proposals will help make our business more successful and sustainable, but 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."
Michael Leahy, general secretary of the Community trade union, said: "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."
A Welsh Government spokesman said: "This is very disappointing news, and a massive blow to those who will be losing their jobs. 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."