Tata Steel could sell one of its Welsh plants in order to get approval for a proposed merger with German steelmaker Thyssenkrupp, BBC Wales understands.
The Trostre steel plant in Llanelli, Carmarthenshire, could be sold as part of an EU investigation into the deal.
The European Commission is investigating the proposal over concerns it could lead to less choice for customers.
Llanelli AM Lee Waters and steelworkers union Community both raised concerns.
The Trostre plant employs about 700 people and produces steel packaging for Tata Steel.
Although it is unlikely the merger will be called off, the European Union is concerned the combined companies could control too large a share of the European steel packaging market, meaning plants like Trostre could be sold.
Lee Waters, AM for Llanelli, said: "Trostre works make high quality products and has a solid customer base. It makes a profit and has a viable future.
"It appears that the decision to offer it up as part of the merger has come from Tata in India, not Europe, and is a cold rebuke to a loyal workforce that have already endured a series of traumas.
"At the moment this is nothing more than a proposal. It has a number of hurdles to jump over before it becomes a reality. But if it comes to that there's strong local management and unions, and a determined community who'll keep supporting the works to find a viable future for tinplate production in Llanelli."
Steelworkers union Community said: "If these reports are accurate the remedies proposal is extremely concerning and confirms that Tata is a junior partner in the joint venture.
"Should the assets in question be sold off the joint venture will become a different project and we need to fully assess what these developments mean for Tata Steel Europe and the UK businesses."
Tata has been approached for comment.