Tata Steel and German steelmaker Thyssenkrupp have both announced that they think their proposed merger is doomed.
Indian-owned Tata runs Port Talbot, Llanwern and Shotton steelworks and the tinplate works at Trostre near Llanelli.
Thyssenkrupp said it expected the plan to turn its steel business into a joint venture with Tata will fail.
The two companies had been trying to get European Union approval. It was feared that concerns about the new venture's size and market dominance would result in a requirement to sell off the Trostre works, where 650 people work.
The Welsh Government has said it has received assurances about the future of the Troste site. It says it has been told Troste would be sold as a operating, profit making site with a viable future.
Steel unions are worried, however, that the Llanelli plant would no longer be a guaranteed customer for Port Talbot steelworks if the two sites no longer belong to the same company.
In a statement, Tata Steel said talks with the European Commission had led both companies to conclude "with deep disappointment" that the merger will not be approved.
Tata said it remains committed to hiving off its European business and will "explore all options" in future.
The steelworkers' trade union Community has called on the company to keep the business intact and called for no "knee-jerk" actions.
Unite the union is calling on the Tata board to give steel workers assurances and end what it calls the "rollercoaster of uncertainty".
In political reaction, representatives of Aberavon have spoken to Tata to arrange an "urgent meeting to discuss the future of steel making in Wales".
Stephen Kinnock MP for Aberavon speaking to ITV News
Member of Parliament for Aberavon, Stephen Kinnock, said "assurances" were needed:
"It is vital that we gain the assurances the local community need from Tata Steel that planned investments in the Port Talbot works will still go ahead, in order to secure the long-term sustainable future of our steel industry".
He added that the company "must urgently make good on their commitments and investment plans, so that the jobs and livelihoods of our steelworkers and their families can be safeguarded".
Assembly Member for Aberavon, David Rees, described today's events as "a cloud over the future of steelmaking in Port Talbot".
David Rees AM for Aberavon speaking to ITV News
Elsewhere Plaid Cymru's Bethan Sayed, AM for South Wales West, said her party had "consistently warned that proposed merger was not a sustainable and secure basis upon which to plan and protect the Welsh steel industry".
She called for an "alternative strategy" to be developed.