Liberty Steel said 250 jobs will go at its Stocksbridge plant, with a further 17 in Rotherham, 15 in Brinsworth and 72 in Newport, Wales.
Miriam Cates, MP for Penistone and Stocksbridge, said: “This is devastating news for steelworkers, their families and our whole community - I will do everything I can to support all those who have been affected by this decision.
“I have spoken to Liberty Steel this morning and I hope to visit the Stocksbridge site as soon as possible.
“It is vital that we do everything we can to support those who are losing their jobs, including looking at what new industries and training opportunities we can bring to the area.
“I will be working closely with the government on this in the coming months.”
Wentworth and Dearne MP John Healey added: “This is devastating news for steelworkers and their families, especially coming so early in the new year.
“Our UK steel industry still needs government backing. For years I’ve been fighting with the industry for help on business rates and UK energy costs which are far higher than our European competitors. The government just isn’t listening, let alone taking action.
“The long-term support the industry needs was discussed when we held the national steel summit here in 2015. It was chaired by Sajid Javid, then business secretary, now chancellor. He pledged to help then but more than four years on we’re still waiting.”