Welsh Country Foods has announced it could be forced to close its lamb processing business after losing its key customer.
A 90-day consultation with workers has started and unions have pledged to do what they can to save jobs.
The plant was purchased by VION Food Group in August 2008. Welsh Country Foods is the major processing facility for Welsh lamb in north Wales, handling 640,000 lambs every year.
Prys Morgan from Welsh Country Foods said it was a "difficult day" having to deliver the news to their "committed staff." He said during the consultation period they will be looking for new customers and other options to save the jobs.
Anglesey councillor Eric Jones said the news will have a "tremendous effect" on the local economy. He said Anglesey has lost a lot of work over the years, but he cannot recall any new jobs coming into the area.
Ieuan Wyn Jones AM for Ynys Môn has described the news as a "massive blow" to the local workforce and the Anglesey's economy.
The former leader of Plaid Cymru says he will be raising the issue with the Deputy Minister for Agriculture Alun Davies AM in the Assembly early next week.
Unite has pledged to work "strenuously" to see if the 350 jobs under threat at Welsh Country Foods can be saved.
Unite national officer, Julia Long described the announcement as "desperately sad news" and said the knock-on effect for the local economy could be "severe."
Jamie Pritchard, the Unite officer at Welsh Country Foods, spoke to ITV Wales after the news broke: