Liberty Steel is set to cut about 350 jobs across its South Yorkshire and South Wales operations in response to "challenging market conditions and a lingering reduction in demand".
The group has taken the decision to reduce its workforce at certain plants to match sustainable production forecasts.
The move follows a review of its UK business in response to "challenging market conditions and a lingering reduction in demand for UK steel products".
Its workforce is set to be reduced by up to 355, with sites in Stocksbridge, Rotherham, Brinsworth and Newport affected.
In response to the job cuts the national officer of the GMB union, Ross Murdoch, said: "Once again we have more bad news for the UK steel industry.
“A country that doesn't produce its own steel for key infrastructure projects and shipbuilding is a country that's destined to fail.
"Our steelworkers deserve better than this.
"The Government urgently needs to intervene and ensure steel companies in the UK receive the backing they have long called for on energy costs and business rates.”