There are fears “thousands and thousands” of jobs could be at risk at Tata Steel in Port Talbot.
Aberavon MP Stephen Kinnock said reported plans to shut two blast furnaces and replace them with electric arc furnaces raised “massive questions” about the future of the steel industry.
The Community Union said it was seeking urgent clarification from the company, and the workforce had not been consulted.
The Tata group was asked to comment.
Mr Kinnock said he had found out through newspaper reports that Tata Steel had reportedly pitched the plan to the UK government.
“They should not be making those kinds of proposals without discussing this first with the workforce and the trade unions,” he said.
“What I would also say is the blast furnaces at the Port Talbot steelworks are an absolutely vital part of primary steel making in Wales and in the UK, and if you don’t have those blast furnaces you’re not able – with an electric arc furnace model – to produce anything like the same quality and variations of steel.
“So this proposal raises massive questions and the future of the industry – thousands and thousands of jobs are on the line.”
A spokesman for Community, the steelworkers’ union, said it was an “absolute disgrace” if these plans had been developed because it said there had been no consultation.
“We are seeking urgent clarification from Tata but rest assured the unions will not accept the end of blast furnace steel production at Port Talbot, which would leave the UK unable to make a range of specialist steels.
“Steel production accounts for half the jobs at Port Talbot and this plan would devastate the town and the community.
“If necessary we will be prepared to fight to protect our members livelihoods and the future of our industry.”
It comes weeks after Tata Steel confirmed it was seeking government support amid reports it is close to securing a bailout worth hundreds of millions of pounds.
The steel industry was suffering before the pandemic but demand has now dived.
The BBC has contacted Tata UK for comment but has not yet received a response.