Unions have claimed Tata Steel was planning to close the bulk of its operation at the UK's biggest steelworks.
Tata's announcement about the future of its Port Talbot plant was anticipated on Wednesday, but it is now not clear when its statement will be made.
Unions have responded ferociously to the proposals after they were briefed by Tata officials.
Tata said it was not in a position to make a statement about its plans.
Previously the GMB said the company would have "fired the starting gun on the death of UK steel".
GMB, along with Community and Unite, promised to oppose the plans with every means at their disposal.
The Labour MP for the area, Stephen Kinnock, said it would be "utter madness" to close the heavy end of the steelworks.
He told BBC Wales that a proper transition plan was needed: "You need a bridge from where we are now to where we want to be.
"Instead of building a bridge with this proposal we were told was coming today, and I'm very glad that it hasn't, they weren't talking about building a bridge, they were talking about putting a load of dynamite under the bridge and blowing it up."
Unite said it was planning a day of action in Port Talbot on Thursday to raise support for its plan to save the steel industry.
The union said more than 50 businesses and community groups would support Thursday's "highly visible and vocal" event, which aims to put pressure on politicians to support measures to back existing steel jobs.
The UK government previously announced £500m to keep open the Port Talbot site, which employs 4,000 people. Tata Steel employs 8,000 people across the UK.
However, the money will see new electric arc furnaces replace existing blast furnaces, reducing the number of workers needed.
The company, which asked the government to provide further funds, is also investing £700m in the site.
Port Talbot's steelworks is one of the biggest polluters in the UK, with its two existing blast furnaces working around the clock.
The new £1.25bn greener arc furnaces are expected to be operational within three years of getting regulatory and planning approval.
The UK government said the deal "has the potential to safeguard" more than 5,000 jobs across the UK.
But the unions have criticised this plan, with GMB's national officer for steel, Charlotte Brumpton-Childs, saying: "Tata must pull back from the brink, work with us to deliver a better outcome for their workforce and protect UK virgin steelmaking.
"If this plan remains unchanged, Tata and UK government have fired the starting gun on the death of UK steel."
She warned closing down facilities while supplying mills with foreign steel would "put us on a collision course with massive industrial unrest".
The unions also want Tata to wait for a report from consultant Syndex, which is looking at other options to decarbonise the steel industry.
Roy Rickhuss, general secretary of the Community union, said: "This is another kick in the teeth for Tata's loyal workforce and the Port Talbot community.
"The unions do not accept the closure of the heavy end and we continue to believe the blast furnaces are crucial to the transition to green steelmaking."
He added the unions would "never accept" the plan and would "oppose it with everything we've got".
Alun Davies, also from Community, said staff had been left "on tenterhooks".
"All we want is meaningful consultation with the company," he said.
Ahead of the expected announcement Tata said it hoped to start formal consultation with staff representatives shortly.
But on Thursday the firm said: "Despite today's press commentary, we are not in a position to make a formal announcement about any proposals for a transition to a decarbonised future for Tata Steel UK.
"We hope to soon start a formal information and consultation process with our employee representatives, in which we would share more details about any such proposals.
"We believe our £1.25bn proposal to transition to green steel making will secure the business for the longer term, bolster UK steel security and help develop a green ecosystem in the region.
"We are committed to a meaningful information and consultation process with our trade union partners and will carefully consider any proposals put forward."