Around 150 workers will be made redundant if the closure of a major Scottish manufacturing site takes place before the end of the year.
Progress Rail Services has told staff it is looking at closing its steel foundry at South Queensferry and move work overseas.
The move would make 147 workers redundant, many of whom reside in the Fife.
Unite the union, which has been given formal notification of the plans, has made a direct plea to the Scottish and UK Governments to intervene to support high-quality jobs.
The union said if the site were to close then it would be another “massive blow” to Scottish manufacturing and the nation’s domestic rail infrastructure.
Unite industrial officer Graeme Turnbull said: “The news that Progress Rail Services is closing its South Queensferry site is a massive blow to Scottish manufacturing.
“Around 150 highly skilled jobs are on the brink of being lost over the coming months, which is why we are calling on the Scottish and UK Governments to immediately initiate discussions with the company and Unite to see what measures could be brought forward to keep the site open.
“It’s depressing news to see a successful UK based manufacturer that has supported this country’s rail infrastructure for over 100 years being faced with closure.
“The loss would mean that the entire UK network would be fully dependant on imported railway components, which would be a further travesty following the closures of Wabtec and the Caley Rail works.”
The South Queensferry facility can produce castings up to five tonnes in weight in a variety of steels from austenitic manganese to Argon Oxygen Decarburisation (AOD) refined stainless steel.
Progress Rail Services acquired the Scottish based manufacturing business that previously traded as North British Steel Group and Edgar Allen Ltd in 2011.
The union claimed the closure follows an indicative decision by Network Rail to award a five-year contract to overseas suppliers.
A Progress Rail Services spokesperson said no final decision had been made on the future of the site.
The spokesperson said: “We have supplied the UK rail industry for more than 160 years with critical track materials.
“Progress Rail was recently notified of Network Rail’s intention to award a significant infrastructure contract to other suppliers.
This award constitutes the majority of the volume Progress Rail has historically received from Network Rail.
“As Network Rail’s largest supplier of track components, Progress Rail is challenging the award and has notified its employees.
“Due to the potential impact of Network Rail’s tender outcome, Progress Rail has opened a collective consultation with employee representatives at each of its UK trackwork locations to discuss possible redundancies and efforts to mitigate potential impacts, although no formal decisions have been made at this time.”