The electronics company Panasonic says 209 jobs could go at its Welsh plants.
The company said 139 jobs in Newport and 70 in Cardiff could be lost. Panasonic said it would cut 15,000 jobs worldwide earlier in February.
Trade union Unite confirmed the company had told it about restructuring plans at the Newport plant, which employs 413 people making telephone systems.
Bryan Godsell, of Unite, said workers were "extremely shocked" by the scale of the possible redundancies.
The assembly government said it was "very disappointing news" and would be meeting the company early next week.
In a statement, Panasonic said 139 positions at its Newport plant where 413 people are currently employed are likely to go.
It blamed "aggressive price competition" as well as the current economic downturn for its decision.
In the Cardiff plant, which has Panasonic's microwave oven division, the company said it planned to reduce production volumes "and consequently there might be a reduction in permanent headcount of up to 70 staff".
The Newport factory makes telecoms equipment for the company's Panasonic Communications Company UK (PCCUK) division.
Mr Godsell, the union's regional officer said the announcement was not "totally unexpected" in the current economic conditions.
"However, our local representatives and members are extremely shocked by the scale of the potential redundancy.
"The only good news is that Panasonic PCCUK Newport is not one of the 27 plants worldwide targeted for closure.
"If the redundancy takes place as proposed it will have a massive impact on the economy of south east Wales.
"Once again we may well see well paid highly skilled jobs sacrificed at a time when the Welsh economy can ill afford further job losses," he said.
He said the union would be working to minimise the impact of the cuts and protect the future of the Newport plant.
Mervyn Burnett of the GMB union said: "This is very disappointing news. We are seeking an urgent meeting with the company to try and find out the nature of the problem."
Jonathan Deacon, of Newport Business School, said the announcement was a sign that the recession was "digging yet deeper" in the Welsh economy.
"It's hitting at exactly what we wanted to do in Wales which was to broaden our base, our economy across the new and high tech-based businesses away from the traditional businesses and industries, and we found we're not safe in even these areas in this particular downturn in the economy."
A Welsh Assembly Government spokesperson said: "This is very disappointing news.
"We have been in touch with the company and will be meeting them early next week to discuss the implications of this announcement and how we might be able to support them."