United Biscuits is to cut the 680-strong workforce at its McVitie's factory in Glasgow by almost a quarter.
The firm said 202 jobs were under threat at its Tollcross site although some could apply for 36 new posts, leaving a net reduction of 166 workers.
United Biscuits said the changes were necessary to keep its operation "modern, efficient and competitive".
The union, GMB Scotland, said the announcement had come at the worst time in the run-up to Christmas.
A United Biscuits spokesman said: "United Biscuits can confirm that it has today announced proposals to restructure working practices and manufacturing operations at our Tollcross site.
"This is part of our ongoing commitment to deliver a modern, efficient and competitive operation, and without these changes Tollcross will not have a sustainable future in the local community.
"We are now discussing the proposal with employees, employee representatives and local government."
The spokesman said the firm had been working with Scottish Enterprise in a bid to safeguard jobs but "regrettably" redundancies would have to be made.
He said workers affected would be able to "benefit from the PACE outplacement programme".
"These major changes unfortunately mean redundancies and 202 jobs are at risk, out of circa 680 roles on site," he said.
"However, it also means some new opportunities will be unlocked with the creation of 36 new roles and also the introduction of 19 up-skilled operator positions available. This would mean an overall headcount reduction of 166 jobs."
Harry Donaldson, regional secretary of GMB Scotland, said: "This has come at the worst time of year in the run-up to Christmas."
Labour MP Margaret Curran, whose Glasgow East constituency covers the area, said the announcement was "a crushing blow to the staff" and a "dark day for the east end".
"Only three years ago the previous owners shed nearly 100 jobs, supposedly to make the factory more competitive," she said.
"If the proposals go ahead it will be a tragedy for those affected and a real blow to the wider east end.
"I have asked for an urgent meeting with the management of the factory, and spoke with officials from the GMB union earlier today. I will be doing everything I can to help people affected by this announcement."
Correction 3 December 2014: The comment from Harry Donaldson of the GMB has been amended to remove an inaccurate quote.