Welsh Country Foods workers plan to drum up support outside Asda in Anglesey and Gwynedd
Unite union members gathered outside Asda supermarkets in Llangenfi, Bangor and Holyhead on Saturday, and handed out leaflets to shoppers.
It follows the loss by Welsh Country Foods of a major contract with the chain, threatening 350 jobs.
Asda said it had met Welsh Country Foods and would consider its decision before responding in due course.
“We want to show Asda that we are not going to put up with its decision,” said Jamie Pritchard, a senior shop steward with the union Unite at the plant.
“They’ve got planning [permission] to build these supermarkets and then they decided to remove the only local supplier of lamb.
‘Decision not taken lightly’
“We are asking the public to boycott Asda stores until it changes its mind.”
Mr Pritchard said the supermarket had acted “irresponsibly in my opinion, putting profits before ethics”.
He added the workers expected a good turnout on Saturday and the public had already sent messages of support to the workers.
A spokesperson for Asda said: “”We had a constructive meeting with Welsh Country Foods.
“We listened to their representations to which we will give full and thorough consideration and respond in due course.”