Tata Steel will start work today on a £240 million project to improve the steel works at Port Talbot, including a re-fit of a blast furnace.
But the company say they also need the help of the UK Government to secure their future.
They warn that Tata may not relight the furnace if support isn't given.
The rebuilding of the blast furnace at Port Talbot is currently in a critical phase as the re-build and assembly process begins.
Tata recently announced reduced shifts and pay allowances for workers because of a fall in orders, and questions are now being asked over whether the re-build will be completed, or whether the company might delay relighting the furnace.
Speaking at the launch today, Tata Steel Europe CEO Karl Kohler said that UK Government policies needed to be changed to secure the future of the manufacturing industry.
Secretary of State for Wales Cheryl Gillan says today marks an 'important milestone' in the work to assemble Blast Furnace No.4 at Tata Steel.
– Secretary of State for Wales Cheryl Gillan
The next four months will see a period of intensive engineering activity as modules weighing over 500 tonnes will be lifted into place over the skies of its Port Talbot site.
Tata is clearly operating in an extremely challenging market – both domestically and internationally - but I am re-assured that Tata is fully committed to its investment in the UK. The company is one of our biggest employers in Wales and I look forward to seeing the new furnace taking shape over the coming months.
Meanwhile Wayne Thomas, a Tata Steel worker at the Port Talbot plant, said workers there were worried about the company's future but were trying to "pull together."