Tata staff in Port Talbot and Llanwern face reduced hours and pay
A third of Tata Steel workers at Port Talbot are facing reduced shifts and pay allowance deductions because of a fall in the demand for steel.
Workers at the sister factory in Llanwern, Newport, will also be hit by changes in their shift patterns.
The company has been meeting unions to discuss a plan about reducing the hours staff work until the market recovers.
A reduction in orders has been blamed for an "extremely tough" situation faced by the company, bosses said.
The changes will come into effect next month.
John Ferriman, managing director of the Port Talbot and Llanwern steelworks said the drop in demand for steel in the UK and Eurozone left them in a situation where some of the production units at the plants were operating at 50% of their capacity.
Mr Ferriman said the company was considering an option to cease 24-hour production in some areas and move to a "Monday to Friday" operation until markets improved.
In June, Tata Steel announced it may delay the start of production at one of its rebuilt blast furnaces in Port Talbot if the steel market remains subdued.
But the firm insists £185m re-building work of furnace No 4 will still go ahead next month.
Tata employs 7,500 people in Wales.
A spokesman for the Welsh government said: "This announcement is a very clear indication of the very difficult market conditions facing Tata and other European steelmakers.
"Despite these challenges, we are pleased that Tata Steel is fulfilling its commitment to its significant investment at Port Talbot, including the rebuilding of blast furnace 4, which will help to secure steelmaking at the site for many years.
"We will continue to do what we can to support the company through this difficult period."
Nick Ramsay AM, the Conservatives' spokesman for business, enterprise, technology and science, said he hoped the workers would be offered support.
"This is disappointing news for the employees affected and their families, although it does further highlight the fragility of the manufacturing industry in Wales," he said.
"I hope Welsh Labour ministers will work closely with Tata and offer what support they can to help protect jobs and create the conditions in which Tata and other manufacturing businesses can thrive."