Almost 250 jobs could be at risk at a packaging factory in Neath Port Talbot. The union Unite said Crown Packaging on Milland Road Industrial Estate, Neath, is considering closing the site due to overcapacity.
Unite Wales secretary Andy Richards said the closure of the company would be a "further body blow to the Welsh manufacturing sector".
The firm confirmed a consultation into the site's proposed closure had begun with the employees.
A statement by Crown said: "We understand this will be a difficult time for all people associated with the Neath factory and we are actively meeting with all employees at the site to explain the proposal in further detail."
Gareth Jones, from the union, said: "Unite has received formal notification that the company is considering closing its site in Neath with the subsequent loss of 245 jobs."
The news was a "devastating blow" to the workforce, Neath AM Gwenda Thomas said.
"'The Box', as it is fondly known locally, has been part of Neath for nearly 80 years and has provided an income for thousands of local families over this time," she added.
Christina Rees, MP for Neath, said the news left her with a "gut-wrenching feeling".
The plant makes lids for cans and has been one of the biggest local employers for 75 years.
A Welsh government spokesman said the "very disappointing" news had resulted from a "downturn in global demand".
Economy Minister Edwina Hart has been briefed by the firm and the union, he added, and the government would liaise with both to see what support it could offer.